I think that a option to reduce or increase sound frequency when playrate is modified would be a good thing.
Let me explain why:
when I chose a play rate of 2X the sound is played 2 time faster. But when the sound is played 2 time faster, the frequency are shifted by 1 octave. so the sound seem shrill. It can be very disturbing.
now, the technical difficulties : for mp3 or any codec based on frequency representation of sound I think that there isn't real difficulties. for other codec ... I don't care
sorry for my poor english skill.