Introduction to Audacity
Incorporating sound into a flash file is quick and simple but what is a bit harder is editing the sounds to fit your animation. This can be done with a sound editor such as Apple's Garage Band or with a free application called Audacity. Audacity can be used to trim, combine, mix and modify sound files and it is a free program available to anyone at http://audacity.sourceforge.net/download/
Once the software is installed you must also install the Lame MP3 Encoder, this will allow you to export your files into an MP3 file which can then be used in Flash. The encoder can be downloaded from this site: http://lame.buanzo.com.ar/ Once this file is extracted you should note where the file has been placed. The first time you export a sound file to an MP3 file Audacity will ask for the location of the lame_enc.dll file. Direct the program to where this file has been stored and then you will be able to export your files as MP3s.
The following tutorials will walk you through the tools and techniques of working with Audacity.
Introduction to Audacity
Complete Tutorial - editing, trimming, adjusting levels, importing music, exporting as MP3.
Wiki Tutorials - many tutorials on a wide range of uses for audacity.Notes on Using Audacity to Edit Music - from the Harker School summer institute