Download Audacity and you can mix music tracks, add sound effects, convert audio files or change the playback speed.
Audacity is a free audio studio for the recording, editing and playing of audio files. It supports the most popular formats (MP3, Ogg/Vorbis, WAV, MIDI, AIFF).
The recording feature enables you to capture all the audio signals and save them to the hard drive. The noise filter can quickly get rid of any disturbances.
Note: A Linux and a Mac version are available to download. For version 2.0.3 another resampler was integrated that's supposed to be faster. Numerous little bugs have also been fixed. All the other changes can be fond in the release notes of the manufacturer.