SoundTouch is an open-source audio processing library for
changing the Tempo, Pitch and Playback Rates of audio
streams or audio files.
The library additionally supports estimating stable
beats-per-minute rates for audio tracks.

- Tempo (time stretch): Changes the sound to play at
  faster or slower tempo than originally without affecting
  the sound pitch.
- Pitch (key): Changes the sound pitch or key while
  keeping the original tempo (speed).
- Playback Rate: Changes both tempo and pitch together as
  if a vinyl disk was played at different RPM rate.

The SoundTouch library is intended  for application
developers writing sound processing tools that require
tempo or pitch control functionality, or just for playing
around with the sound effects.