Audacity is a free cross-platform multitrack audio player/recorder/editor. It's available under
GNU General Public License, which means you can view and modify the source code as well. The
GUI is similar to that of the free version of
Pro Tools (5.0.1 I believe) which makes it very
user friendly.
Mp3,
WAV, and
AIFF file formats are supported for both importing and exporting. Audacity also has a nice selection of
digital effects. For the present, though, it is still under
development so for now it's best used as a player/editor rather than a multitrack recorder. Alternative tools for
Linux sound recording are
ecasound (best with a GUI) and
GLAME.
http://audacity.sourceforge.net