Libsonic is a free C library for speeding up or slowing down speech. However, it's optimized for speed-ups of over 2X, unlike previous algorithms for changing speech rate. Sonic enables clear playback of audio books at 3X speed, and some blind listeners are able to listen at 6X. Libsonic has also been integrated into several text-to-speech engines. It works well on all platforms, even Android phones without FPUs. Pitch control is also supported.