I bet that any delays on the sound due to hardware will happen on both audio streams at the same time and speed, so by setting both positions to 0 should give the same delay on every system, depending on the length of the song.
Of course the playback speed will remain constant, but since the audio position resets are triggered from a separate thread, and not simultanely, I'm sure the delay is not a constant, since the game speed itself isn't. In fact, in all my games I have to compensate for a MMF2 design mistake, which directly involves a delay between multiple sound actions, that varies per system and CPU usage, and is very closely related to this (though it's about modifying the volume of a sound). Luckily, I know a workaround for that.
However, I'm positive that the delay isn't constant. Seriously, I don't make that kind of mistakes, I know MMF2 inside and out.