AUDACITY - the star software on sourceforge//very good sound editor

A fast multi-track audio editor and recorder for Linux, BSD, Mac OS, and Windows. Supports WAV, AIFF, Ogg, and MP3 formats. Features include envelope editing, mixing, built-in effects and plug-ins, all with unlimited undo.
Learn to install Audacity, plug in your microphone, record your first podcast, and get it online as quickly as possible. This eBook also covers podcasting-specific topics, such as adding background music or conducting interviews, and includes lots of tips and trickts to make complicated editing even easier.
Download available on Sourceforge