Robin Good: Here is a useful guide to 25 free audio editors, both for PC and Mac, which can solve most of your problems when it comes to audio editing, conversion and recording.
The mini-reviews are not super, but the collection per se is quite useful indeed.
The guide also includes some nifty digital instruments and tools like Hydrogen (for Linux) fro drumpad programming, the Kangas Sound Editor to create music and sound effects, AudioCutter which is very useful to split large MP3 files.
P.S.: Unfortunately the short reviews (some of which are a bit incomplete) fail to specify which apps are for PC and which for Mac, but you can derive - in many cases - it by looking at the screenshots