The rhythm of the music is defined by the timer on the sound chip which is part of the neo geo board. That is controlled by the Z80 which is also part of the neo geo board. The multi cart only provides the program for the z80 to run. I suggest you try the cartridge on at least one different MVS...
As I said privately to Darksoft on discord, this must have to do with the long traces that come from the CIF custom IC and go through the board up to the CN2 on the opposite side of the board.
The simms are flashable, they will hold whatever you install from the right CD. The two different ones that are close to the cart slot who keep program data. What is written to these two simms are what differ from one cart type to other.
Some games were rental or sample and therefore do not offer freeplay mode. Example, all Brazil games were (supposedly or so Japan Capcom wished them to be) rental and lack freeplay option and do display their digital coin counters when you press action buttons while on atract mode.
That is the point of the mentioned audio fix. The cps2multi introduced glitches on the PCM data being send to the A board. That's why it needed a modification.
You asked what/where the sound was generated and I answered. It is generated on the A board with data from the ROMs.
it is on the A board, the problem with the multi is that one of the graphics buffers drains some current that causes the audio buffers to malfunction, the A board get feed glitched PCM data which is reproduced at the speakers. The fix solves that issue
DEX and CEX used different challenge keys because they wanted to make it so MC installed KELFs for DEX consoles not function on CEX consoles and MC installed KELFs for CEX not function on DEX console... But they still needed the same memory cards worked on both so it has a command to swap...
Even on the earliest CPS2 multi boards, the jumpers never had any effect to begin with. What the jumpers do is select the functionality of the PIN 32 between A20 (27C322) or byte (27C160). That does not affect the multi at all.
When I added the COH keys to PS3MCA, I noticed that the dongle does not even acknowledge this additional command (because it does not support two keysets like the normal retail memory card does) so it worked either way, the command has no effect on it.
What the command does is tell the memory...
They do on the expansion port. The thing is, when they designed the Master System they just made the expansion port and the cart slot the same thing. The Japanese SMS had to keep the original cart slot design because it need to be retro compatible with SG-1000 stuff.