Sets the current playback rate of the specified instance, with 1.0 being normal speed, 2.0 being double speed, and so on. Not all devices are guaranteed to support modifying playback rates. Use GetSoundMaxRate and GetSoundMinRate to check the valid ranges for the current device, any values outside this range will be clamped. The instance ID is the value returned from PlaySound when you initiated the playing of the sound.
SetSoundInstanceRate( iID, rate )