"Stretch" length of sound file?

Can Unity "stretch" the length of a sound file by playing the file for a longer period of time than the clip length?

Sort of. AudioSource.pitch is a misnomer. AudioSource.playbackRate would be more accurate. Of course, the two things are tied together, so every time you increase or decrease by a factor of the 12th root of 2, you'll have changed the pitch by a semitone. Unity has no formant-preserving or "time-stretching" capability.

I don't think Unity has a Rateshift capability. The only thing to lengthen the sound may be through AudioEchoFilter. But I might be wrong...