Audio recording devices range from tape-based recorders such as micro, mini, and standard audio cassettes and reel-to-reel tape recorders, to digital audio recorders that record on compact discs, hard drives, and media cards.

For this reason, if the original is in a digital format, any copies need to be made in a comparable digital format that uses lossless compression. Preferably, the recording method used to duplicate the original would have no compression at all.

On many digital audio recorders, deleted files can be recovered because the media is treated like a computer hard drive, as long as the files have not been overwritten. Your expert will need physical access to the device in order to recover any deleted audio data, if it still exists. Overwritten analog audio tapes cannot be recovered.

It is becoming increasingly popular to recover deleted audio recording from smart phones. If you are able to get access to the physical storage on the phone, such as a media storage card, then recovery of audio files is very possible.

If the audio evidence is on an audio tape, such as a cassette tape, any playback of the audio recording will cause the tape to deteriorate. You can think of a cassette tape as a miniature VHS tape. They both deteriorate whenever they are played, rewound, fast-forwarded, or paused. The original audio tape should never be worked from, just as the original hard drive in a computer forensics case should never be used for analysis. A duplicate recording should be made on another cassette tape, or in the best circumstance, a digital copy should be created.

Another issue with magnetic tape is that the tape itself is made from plastic coated with a metal oxide and the plastic does stretch a tiny amount each time the tape is played. This stretching that occurs as a result of repeated playing of a cassette or other audio tape will result in distortion of the recording over time and can cause the pitch of the audio to change.

If only the audio recording itself is received in discovery, critical information can be missed that might reside on the audio recording device itself, such as the time and date the device was set to when the recording was made.

If the audio is being collected from a smart phone, the date and time information may be available as part of the file system metadata on the phone. This is also true of most digital devices that have a time stamp feature, including digital answering machines, and digital pocket recorders.

The Apple iPhone stores voice mails on the phone and the voice mails can be obtained, including the time stamps, by performing a backup of the phone and then analyzing the backup data itself.

If possible, you want the audio recordings produced in discovery exactly as they were originally collected. Just as with digital video recordings, digital audio recordings can lose quality through file conversions. If this is not an option, you want the audio recordings in a format that is easily accessible using normal audio playback software programs, such as Windows Media Player or iTunes. You do not want the audio recordings to be dependent upon a single piece of audio playback software. A common scenario is that the audio is produced, but is dependent upon a single software program to be played at all, and sometimes these software programs will not allow for rewind, fast-forward, and pause functions. This occurs when a law enforcement agency produces the audio in an encrypted format that cannot be converted into a standard format and must be played using the supplied player only. This forces you to listen to the entire audio recording even when only a small section may be of interest.