The problem with implementing this is that once a file is decoded, LÖVE doesn't distinguish between MIDI/tracker music (with which it's trivial to change the playback speed) and standard audio formats (with which it isn't, at least not without changing the pitch too). Everything's decoded into raw SoundData. You can use Source:setPitch to speed up/slow down playback, but again, that raises/lowers the pitch too, which is not necessarily ideal.
I agree it'd be a nice feature to have, but implementing it for MIDI and not for raw audio is impossible to do with LÖVE's current architecture. Any suggestions as to how to implement it for raw audio, however, are very much welcome (and should get their own forum post/ticket).