Sega Genesis Model 1 iffy sound channel

I am not inside the Genesis/Mega Drive as often as other consoles, so I am hoping some of you Genesis mechanics with hands-on knowledge can mention any specifics to check out for this situation.

Genesis Model 1, VA 6.5.

Streets of Rage as an example - the music of the first level has a pounding bass drum and a snare drum lead-in. I played the game (via the Genesis 6-Pak cart) after installing a preamp stereo mod and the bass and snare were 100% missing. Not low volume - they were just gone. Power cycled and they came back. Played a bit. Power cycled. Fine. Power cycled. Missing.

Today - Cold boot. Missing, but I gamed a bit... after hitting the first dude, the channel roared back to life and remained.

If you load the game in a debug version of Regen (emulator) and disable the last channel of the YM2612 (channel 5), it is basically what is happening to me.

I am not really familiar (on a design and circuit level) as to how the YM2612, Z80, 68k, SN76489, etc. come together to form the Voltron that is Sega Genesis audio. Ideally, it is just an old filter capacitor that needs replacing for sound that originates somewhere and gets mixed in.

I am going to dive into trace inspection, schematic research, and all of that - but I was wondering if you guys have encountered this before... prior to me reading Genesis schematics for the first time. Appreciate it.