WaveShop - Open source audio editor
Super Sound - Super Sound is a resourceful audio and music editor.
TwistedWave - TwistedWave is one of the unique audio editors that allow you to edit your music file online without any installation.
Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Add Audio to Video - Add Audio to Video empowers you to add custom audio tracks to your video files.
SunVox - SunVox is a small, fast and powerful modular synthesizer with pattern based sequencer (tracker).