You need to check out Audacity.
It’s open source, it’s free, and it’s pretty powerful. It supports multiple audio files, allows you to go in and edit them with a fine tooth comb, apply filters as well as multi-track mixing.
If you’re not ready to spend the big dollars on a professional software solution, Audacity will kick some major butt for next to nothing whether editing a podcast, trimming an mp3 or tweaking some music or voice over.
Read more about Audacity at Wikipedia.
Download the latest version of Audacity at http://audacity.sourceforge.net/.