http://bugs.winehq.org/show_bug.cgi?id=3930
--- Comment #38 from Jörg Höhle hoehle@users.sourceforge.net 2010-03-04 04:09:37 --- W98SE executes both WAVE and MIDI callbacks from the same thread as seen from WineTestBot job #938. https://winetestbot.geldorp.nl/JobDetails.pl?Key=938&log_201=1#k201
So this thread is either an OS-wide callback dispatcher, or a per-app callback dispatcher, but not the app's nor the main winmm thread if such exists (since SuspendThread is said in comment #18 to not make it hang) -- or something else entirely.