As far as editing audio I'd reccomend Audacity.
www.audacity.sourceforge.netIt's a free program that has an incredible set of audio tools/filters, etc. including an equalizer that can have as many buttons as you want, precisely where you want. It will not export as an MP3 without an additional piece of software. I have a WINK tutorial I developed for the NDAA at the Courtroom Technology seminar two weeks ago that I can send you.
I mentioned the equalizer because that is one of the easiest and most non-invasive methods of "cleaning up" audio. By non-invasive I mean you are not really changing anything, just adjusting the levels of sounds already in the recording. Along this line I'd agree with Bradley to a poin; if you are going to apply sophisticated filters/enhancers and make clear audio out of background noise, you need an expert to discuss what he/she did. However, if you had the original cassette tape and played it on a boombox with an equalizer, there is no prohibition against bumping up or dropping down the treble. With the full version of Realplayer as well as Quicktime and Windows Media Player, you have an equalizer available on your computer to do the same thing. Audacity has a multi-band equalizer, but actually applies the equalizer settings to the audio as an enhancement, not as a playback feature.
If you are actually going to apply filters to an audio recording you should not have a problem if you make clear that the original is available and both the original and enhanced version has been provided to defense counsel so that they had time to formulate objections. The defense needs to object to the accuracy of a specific section, because the predicate that can be laid for the enhanced version is the same as the predicate for the original. If you have an eyewitness to the sounds, they can just as easily testify that the cleaned up version is what it purports to be as the original was, if not moreso. You have a bit of a problem if you are admitting audio from an event that was not personally heard by a witness. The predicate would be the same, but this person cannot say that the tape is accurate, only that the tape is what the equipment recorded. Then you might need an investigator to testify that they muted the 8K frequency and double amplified the 1K band. Again, have your original available and provide both to the defense well ahead of time so that they can make their specific objections at the time the evidence is offered or even prior to trial. The judge can listen to the two recordings and determine whether there is any merit to the defense contention that the cleaned up recording is inaccurate.
This is also the same procedure you would go through to admit an audio that eliminates "dead time." Make you sure you have admitted your original and then make clear you are admitting and publishing a copy of what's already in evidence that will aid the jury in reviewing the evidence. That, or digitize your recording and simply advance it during playback, reading into the record where you are stopping and starting, to the precise point that the good stuff appears.
i can send out more materials on using these programs if anyone needs them.