Digital Projects Toolkit

Windows/Mac/Linux - free.
Audacity is a free, open source audio software for multi-track recording and editing.

Mac - pre-installed.
GarageBand lets you create and record music. It has a vast library of sounds for you to use in creating audio tracks.

Windows/Mac - free for non-commercial use.
WavePad is a full-featured audio and music editor that lets you record and edit audio tracks. The free version is available for non-commercial use only.