libsoundtouch

SoundTouch is an open-source audio processing library for changing the Tempo, Pitch and Playback Rates of audio streams or files:

Tempo (time-stretch): Changes the sound to play at faster or slower speed than original, without affecting the sound pitch.
Pitch (key) : Changes the sound pitch or key, without affecting the sound tempo or speed.
Playback Rate : Changes both the sound tempo and pitch, as if an LP disc was played at wrong RPM rate.