-Super Mario Land 2 - Pops in menu are fixed by immediately updating APU output after a write
-Perfect Dark - Low voice volume is fixed by having the correct output when the channels are disabled (but DAC is still enabled)
-Daiku no Gen - Low voice volume is fixed by keeping square channel output to digital 0 (= analog 1) until its first tick after being enabled (the game does not let the channel tick at all while the voice sample is playing)
Based on Megaman 2 and Taiyou no Yuusha the envelopes shouldn't be ticking when period is set to 0 (otherwise the sound in these games breaks - especially the noise channel)
This seems to contradict old documentation that says a period of 0 is treated as a period of 8, but test roms don't seem to validate either behavior and this fixes the audio for both games.
