Sonic is a simple algorithm for speeding up or slowing down speech. However,
it's optimized for speed ups of over 2X, unlike previous algorithms for changing
speech rate. The Sonic library is a very simple ANSI C library that is designed
to easily be integrated into streaming voice applications, like TTS back ends.

The primary motivation behind Sonic is to enable the blind and visually impaired
to improve their productivity with open source speech engines, like espeak.
Sonic can also be used by the sighted. For example, Sonic can improve the
experience of listening to an audio book on an Android phone.

WWW: https://github.com/waywardgeek/sonic