Add the possibility to slow down or speed up audio files

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1. Speed up the Sound in an Audio File

To speed up or slow down the sound of a file, use speed to modify the pitch and the duration of the file. This raises the speed and reduces the time. The default factor is 1.0 which makes no change to the audio. 2.0 doubles speed, thus time length is cut by a half and pitch is one interval higher.

Syntax: sox input.wav output.wav speed factor

$ sox input.wav output.wav speed 2.0

http://www.thegeekstuff.com/2009/05/sound-exchange-sox-15-examples-to-manipulate-audio-files/