Tracking issue for basically this comment:

We're already doing this with BASSWASAPI, so we should have everything we need in place to do the same for regular BASS playback without much effort.

It is unclear how exactly BASS handles parallel playback of independent channels (i.e. what we're currently doing). If they are kept separate (= multiple streams to the OS mixer), then we might have to watch out for some minor changes in offsets. But observing its winapi calls (using api monitor) shows no increase in accesses to wasapi buffers when starting additional channels, so I think that BASS is essentially already using something akin to a global mixer anyway.

Currently blocks #705 , #6650, and ppy/osu#36296