Most of the synchronization was too deep in the implementation so that it did not guard everything it needed.
Now each song has precisely one mutex which must be locked for all access to its internals - this is done in the public ZMusic interface
It is now being handled by the controlling code.
While of no benefit for GZDoom itself, this finally allows to separate the entire music code into a separate, engine independent project that merely provides streamed music data when not playing on a hardware device (WinMM Midi or CD Audio.)
The tight coupling of the music code with the sound backend made this nearly impossible before
