I think CoolEdit, 2000 or Pro - prefferably Pro, is an excellent sound studio. Unfortunately, the company was bought out by Adobe I think. There used to be some old shareware versions of each; they were fully functional, but only let you do 1 type of filtering per session. You'd have to record a sound and do all echo filtering (for example), save, quit, open again and do compression filtering afterwards (for another example). You could probably search some file sharing program like Kazaa (Lite) for the sharewares. They're truly worth the searching.
Not sure how functional the demo of this is, but here's the program CoolEdit has become after the merger:
http://www.adobe.com/products//audition/main.html