So, in the next nightly we should be able to control music volume, loop status, position etc in real time. Due to the way sound and music are set up internally, it is not entirely straightforward in the current implementation and it is possible that a better way to handle this exists, I just couldn't see it. Having said that, it's not difficult either, thankfully.
. More specifically, if I want to adjust on the fly, say, the volume of the music currently playing, what I would do is something like this:
myMusicSoundSource = Sound.musicSoundSource();
myMusicSoundSource.volume = 0.4;
If the music file is a 44KHz mono ogg, we can also do stuff like
myMusicSoundSource.position = [-1,0,0]; // throw the currently playing music to the left channel