PCM volume, channel, and quality controller.
FireRed: 0x1DD0C8
Emerald: 0x2E0104
Vanilla value is 00 C5 94 00, what does they mean?
C5 can take two parts. C is PCM volume, 5 is PCM channels. Some hack increased channel to 12, so it would be CC.
94 can take two parts too. 9 is DAC bit (9 is 8-bit), on emulator it won't be noticed difference except E or F, PSG audio will quieter, especially Noise channel, some music use it as drum, just don't modify it. 4 is PCM quality, 1 is worst, C is best, E and F are laggy thus can't listen.
For those four value, 0 is not lowest, but somehow default. Volume is F, channel is 8, bit is 9, quality is 4.
Is it possible to improve PCM quality so music would listen better? Partially, unforunately.
For vanilla FR/EM soundfont, plus any hack uses vanilla one, increase PCM quality will make drum speed and pitch up.
For BW soundfont, just a few hack can take advantage of high PCM quality. Unbound OK, it even give an option to adjust quality (2 4 8). GS Chronicles OK. Though not suitable for playing game, because high PCM quality will cost more CPU cycle thus game will more laggy, it extremely suitable for GSF ripping. One more weird thing, 9~C & E~F will stuck at white screen. Other hack can't increase beyond 4, such as Saiph, Sors, Saiph 2, etc., really shame.
For custom soundfont, like Clover and SOTS, if increase quality won't distort drum, then it can be improved.
For Decomp Emerald hack, try searching 00 CC 94 00 to find this address.