I'm not trying to be negative, but will write my thoughts: I think that it went in bad direction. Idea of using YM and DMA at once for sound seems for me as bad. I looked couple quality games: Magic Boy and Lethal Xcess , and both have only DMA audio on STE (Magic Boy will have YM on ST) . Both work with only 512KB RAM . So, I really don't think that background music in levels must eat lot of CPU power and/or RAM . To solve DMA music should not use some stand-alone MOD player code, which can use 100% CPU power, since it does nothing else, but preparing such music data, what can be played without need for mixing data and similar. So, music playback code should only control addressing and looping of DMA - what surely needs not more than 1 % of CPU power. The problem here is, as I see that STE DMA is with interleaved L-R channel, so you can not simply use music on 1, and effects on other channel. Some CPU time must go in mixing effects into music data, then restoring music data . Simple copying of 10 KB needs about 6mS. With some mixing more. Surely not easy to code.
Another way would be to silence complete YM music data - not on fly, but before assembling
But before all it, what about Falcon fixed version with YM audio ?
Famous Schrodinger's cat hypothetical experiment says that cat is dead or alive until we open box and see condition of poor animal, which deserved better logic. Cat is always in some certain state - regardless from is observer able or not to see what the state is.