Audacity

Audacity is an audio editing/recording program for mac, windows, and linux. it provides a very wide set of tools to make your audio sound awesome. You can layer sounds on top of each other, cut up sounds, and you can change the volume to make it louder or softer. The best thing about Audacity is that it's free!

Audacity Interface

Audacity website
http://audacity.sourceforge.net/