May I interrupt this technical discussion to state that "Champion Boat" on the 220-in-1 multicart must have the worst controls of any racing game that I have ever encountered.
Here is my current emulation of Mapper 296's sound. It does not sound bad at least, but I do not have enough footage of the 220-in-1 multicart to judge whether the sampling rates and volumes are correct. In particular, Cut Fruit sounds a bit high-pitched? Right now, I am using the sampling rate from $4010 and the value written to $4031 and $4032 as a volume with range 0-128. This is hacky because 220-in-1 sometimes writes nothing and sometimes writes $00 to $4010, which I am replacing with $0C.
My progress was hindered by what I think must be a mistake in FC Pocket 600-in-1's code that (I think) queries the wrong $4014 bit to check whether the song has finished playing and if so, restart it again. Because it queries the wrong bit, it keeps restarting the song during every vertical blanking interval. The song is not actually restarted at every vertical blanking interval only because I am blocking access to the sound channel registers until it is either disabled by writing $80 to $4033 or it has finished playing by itself. (If I did not block them and merely flipped the $4014 bits, then 600-in-1's menu song would restart after the language selection --- which it does not do,
according to this video) --- and more importantly, I could either get 600-in-1 or 220-in-1 working, but not both.)
After davidstech's console arrives and he tells me (or posts samples) of what still sounds wrong, I will then make a class out of the current sound emulation and post the source code so he can incorporate it into MAME.
