The only sure way to avoid destruction of the audio file is to upload it live to a source to which the on-scene officers have no access.
This can be a commercial service like Qik, a private broadcasting system modeled after Porc-411, or even your home answering machine. In this age of VOIP and virtual phone numbers, you can even set up your own private line that will instantly answer and start recording.
On-body DVRs that record continuously have their place, and will continue to do so, but it's a pain to carry yet another piece of hardware around. A portable MP3 player that can also record is probably your best bet, because it doesn't automatically raise suspicions.