Audio Chain of Custody: A Forensic Checklist

Audio chain of custody has become a bigger issue for prosecutors, defense counsel, and records staff as the volume of recorded evidence keeps climbing: body-worn camera audio tracks, 911 calls, jail calls, interview room recordings, and voicemail or cell phone audio pulled into a case file. Every one of those recordings can be challenged on the same basic ground, regardless of what it captures: can the party offering it show that the file played in court is the same file that was originally recorded, handled by known people, in a documented and verifiable way, from the moment it was captured to the moment it was offered as evidence? That question is what audio chain of custody is built to answer, and the Scientific Working Group on Digital Evidence (SWGDE), the standards body most forensic audio labs look to for practice guidance, lays out exactly what a defensible answer requires in its Best Practices for Forensic Audio (document 08-A-001, version 2.5, finalized June 9, 2022).

What Audio Chain of Custody Actually Means

Chain of custody is the documented history of who had possession of a piece of evidence, when, and what was done to it at each step. For a physical exhibit like a weapon or a drug sample, that history is mostly about hands and storage. For a recording, audio chain of custody has to account for something slightly different: not just who held the device or the disc, but what format the audio was in at each stage, whether it was copied, converted, or transcoded, and whether a cryptographic hash confirms the working copy matches the original bit for bit.

SWGDE's guidance treats this as a connected sequence rather than a single event. An unbroken audio chain of custody typically has to show:

• What was originally recorded and on what device, whether that is a body-worn camera, an interview room system, a 911 recording platform, or a personal phone.
• How the recording was retrieved from that original device or media, and whether the method used preserved the audio in its native format.
• Who handled each copy, including submitters, evidence custodians, and examiners, with dates and a description of what each person did.
• Whether the recording was altered at any point, whether through transcoding, enhancement, redaction, or format conversion, and what was documented about that change.

Skipping any of these links does not automatically make a recording inadmissible. Courts applying Federal Rule of Evidence 901 generally hold that gaps in a chain of custody go to the weight a jury gives the evidence rather than to whether it comes in at all, provided the proponent can still show the recording is what it claims to be. But a thin or undocumented audio chain of custody hands opposing counsel an easy opening, and it can turn a straightforward recording into a drawn-out fight over foundation.

Why Courts Care About Audio Chain of Custody

Federal Rule of Evidence 901(a) requires the party offering a recording to produce evidence sufficient to support a finding that it is what the party claims it is. Rule 901(b)(9) specifically allows authentication through evidence describing a process or system and showing that it produces an accurate result, which is precisely what a documented audio chain of custody is built to provide: a repeatable account of the process the recording went through, not just a witness's say-so that the recording is genuine.

That standard matters more, not less, as recordings pass through more hands and more software before they reach a courtroom. A 911 call might be exported from a dispatch recording platform, converted to a different format for the records request, then redacted before release, then enhanced for trial. An interview room recording might move from a proprietary interrogation system to a records management database to a prosecutor's case file to an expert's workstation. Each handoff is a link in the audio chain of custody, and each one is a place where an undocumented change, a lost original, or an unexplained file conversion can become the subject of cross-examination.

Evidence Retrieval: The First Link in the Chain

SWGDE's guidance is explicit that the strongest audio chain of custody starts with requesting the original recording, or the earliest generation available, rather than a copy of a copy. An original digital recording is a bit stream from which the acoustic signal can be generated, and because digital copies can be made exactly, a bit-stream duplicate verified by a hash function is treated as equivalent to the original. The guidance ranks retrieval methods roughly in this order of preference:

1. The original media, such as the memory card, disc, or cassette the recording was made on.
2. The original recording device or system, which preserves native file formats, timestamps, metadata, and the recorder's own internal clock.
3. A forensic image of the original storage media or files, made with tools designed for that purpose, which preserves the audio stream, metadata, and timestamps even when the original device cannot be retained.
4. A file transfer verified by a hash value, which confirms the copy matches the source even when the original media is not available.
5. A transcoded or analog transfer, used only when none of the above options exist, and documented along with its limitations.

Whichever method is used, the goal is the same: minimize the number of conversions between the original recording and the working copy, because every unnecessary conversion is a potential point where the audio chain of custody is weakened and where quality can be lost in ways that are difficult to reverse later.

Documenting, Marking, and Storing the Recording

Once a recording is in hand, SWGDE's guidance calls for a specific set of documentation steps that, taken together, form the backbone of an audio chain of custody record:

• Inspect and document the physical condition of any media received, including damage to a cassette housing, scratches on a disc, or signs of water or contaminant exposure, with photographs where damage is present.
• Record the hash value of every digital file at the point of receipt, so the file can later be verified against that same value to confirm nothing changed.
• Uniquely identify and mark each item consistent with laboratory policy, using methods appropriate to the media; for example, writing only on the non-data hub area of a disc with a disc-safe marker, or marking a sealed container rather than a memory card too small to label directly.
• Document the state of write protection on arrival, and activate it if it was not already engaged, noting the reason if it is left off for a specific technical reason.
• Log every transfer with shipping documentation, tracking numbers, and the dates sent and received, so the custody record accounts for the recording's location at every point in its history.
• Store the recording under conditions appropriate to the media, away from sources of electromagnetic interference that can affect magnetic tape, and within the temperature and humidity ranges the equipment and media manufacturers specify.

None of these steps requires specialized forensic software. They require discipline and a consistent process, which is exactly what makes audio chain of custody something an agency, a law firm, or a records unit can build into a standard workflow rather than treating as a one-off task for complicated cases.

A Practical Audio Chain of Custody Checklist

Attorneys, investigators, and records staff who are not forensic examiners themselves can still protect the audio chain of custody for a recording before it ever reaches a lab, by working through a short checklist:

1. Identify the original source. Confirm which device or system actually made the recording, and ask whether that device or an earlier-generation copy still exists before accepting a secondhand copy.
2. Request the native file, not a re-recording. A recording exported directly from the source system, or copied by file transfer, preserves far more than a recording made by holding a phone up to a speaker or playing audio over a conference line.
3. Capture and record a hash value as soon as a digital file is received, and store that value alongside the file so it can be checked again later.
4. Write down every person who touches the file, including the date, what was done, and why, whether that is a transfer, a format conversion, an enhancement pass, or a redaction.
5. Keep the original separate from any working copy used for enhancement, redaction, or courtroom playback, and never perform irreversible processing on the only existing copy.
6. Avoid unnecessary format conversions. Each conversion is a decision point that belongs in the documentation, and converting a file more times than necessary multiplies the number of links in the audio chain of custody that someone may later have to explain.
7. Confirm the final chain of custody record travels with the evidence, not just in a separate case file that might not reach the person offering the recording in court.

Our video evidence chain of custody checklist covers the same discipline applied to video files, and the two checklists overlap heavily wherever a recording captures both audio and video, such as body-worn camera footage or an interview room system.

Common Mistakes That Break an Audio Chain of Custody

Most gaps in an audio chain of custody are not the result of misconduct. They are the predictable result of rushed handling, well-intentioned shortcuts, or simply not knowing what to document. The most common problems include:

• Re-recording instead of transferring. Playing a recording over a phone's speaker and capturing it with a second device degrades the signal and introduces a generation of copying that cannot later be verified against the original.
• No hash value recorded at intake. Without a hash taken at the earliest point possible, there is nothing to compare a later copy against if its integrity is ever questioned.
• Processing the only copy of a recording. Running noise reduction, trimming, or redaction directly on the sole existing file removes the ability to independently verify what the original actually contained.
• Undocumented handoffs. A recording that moves between a records unit, outside counsel, and a forensic examiner without a dated log of each transfer leaves a gap that is difficult to fill in after the fact.
• Treating enhancement or redaction as outside the chain. Any process applied to a recording, not just its physical handling, belongs in the audio chain of custody record, including the settings used and who approved the result.

Our guide to what forensic audio enhancement can and cannot do and our law enforcement audio redaction guide both depend on the same underlying discipline: the original recording has to be preserved and documented before any processing happens, so the processed result can always be checked against it.

Conclusion

Audio chain of custody is not a separate legal requirement layered on top of forensic audio work; it is the documentation that makes forensic audio work usable in court at all. SWGDE's Best Practices for Forensic Audio lays out a consistent process for retrieval, documentation, marking, and storage that any agency, law firm, or records unit can adopt, regardless of whether a formal forensic lab is involved. Recordings that start as native files, carry a hash value from the moment they are received, and come with a dated record of every person and process that touched them are recordings that hold up when the chain is tested.

Focal Forensics handles audio evidence under the same chain-of-custody discipline described in SWGDE's guidance, with hash verification, detailed processing logs, and the original recording preserved alongside any enhanced or redacted result. Learn more about our forensic audio services or reach out to discuss how a specific recording's history should be documented before it goes any further in your case.

Phone: 303-900-3585 · Email: info@focalforensics.com

Sources

• Scientific Working Group on Digital Evidence: Best Practices for Forensic Audio, SWGDE 08-A-001, Version 2.5 (June 9, 2022)
• Cornell Law School Legal Information Institute: Federal Rule of Evidence 901, Authenticating or Identifying Evidence
• Scientific Working Group on Digital Evidence: Best Practices for Forensic Audio, document listing