Two possible approaches come to mind.Import the music as a sound
You can import the music into the sound folder, and play it as if it were a sound, which would cause it to behave like a sound file for playback rate purposes. This also means the music would not be streamed though, so the end user would have to wait for the song to fully download before the game would start. If the song is short that shouldn't be that much of a problem, and if your distributing to desktop it's really
not a problem. This is probably worth trying at least, to see if it adds an unacceptably long wait to the loading time of the game. My guess is that it won't change the loading time very much, as long as it's not a 10 minute song. Multiple files for each music variation
If the music only ever has to play at the original rate and one other rate, (e.g. for a MarioKart normal-lap vs final-lap type of situation) then you could actually save a second copy of the music at the alternate rate, and include both copies in your game, switching to the alternate as needed.
Obviously this method is not so good if you want a smooth transition of the playback rate, or a bunch of different playback rates.