v073 has some shortcomings in MSU1 behaviour, most notably it ignores
the DSP mute flag (which will cut off external audio on a real SNES).
Hack authors stumble upon those when running on real hardware for the
first time after testing on less recent bsnes versions.
Backports from higan v094:
- audio error flag
- init behaviour (zero out MSU1 regs)
- initial volume
- DSP mute flag