I've been reading Unity's manual on the
AudioManager's properties. Speed of Sound is the only undocumented option. What is it for?
It looks like this value is a measure of the physical speed of sound, and likely plays a part in the result of the simulation of the Doppler effect (if enabled).
The value shown in the screenshot in the documentation is 347, which is very close to the actual speed of sound (343 meters-per-second at sea level).
It doesn't appear to have anything to do with the observed playback speed of the sound.