@ckie uhhh stemmy isnt open source and i have no plans to do that any time soon either ^^'
the reason why they arent synced is that a lot of times, people dont provide aligned stems. like they provide a 30 sec audio for a lead, a 2 min audio for the drums, and a 10 sec audio for the ending effects. if you play them all at once, its not gonna sound good, and if i provide a button that does that, and it sounds bad, it reflects poorly on both my website and the uploaded song. i hope that makes sense!
you can probably write a couple line script to run on the console that .play()s all the audios on the page if you really need this