LMMS - Make music with a free, cross-platform tool
Super Sound - Super Sound is a resourceful audio and music editor.
GarageBand - GarageBand is a fully equipped music creation studio for Mac
Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Ableton Live - Ableton makes Push and Live, hardware and software for music production, creation and performance. Ableton´s products are made to inspire creative music-making.
Add Audio to Video - Add Audio to Video empowers you to add custom audio tracks to your video files.