Some games only boot after the "9" key is pressed to reset them (e.g. Gaiares), various games need "9" to play sound (e.g. Puyo Puyo, Devil Crash, Raiden Trad, Sonic games with Knuckles, voices of Mortal Kombat 2 and Moonwalker), various have horrible sound, seems like playing wrong data (like Shadow Dancer when start 1st stage, Golden Axe 2 or JuJu Densetsu when an enemy dies - voice related?).
SVN rev. 3957 seems ok. SVN rev. 3958 is the 0.128u7 merge. SVN rev. 3963 made some changes to genesis.c related to Pico driver. SVN rev. 3964 fixed the drivers requiring cartridges, when a regression in src/mess/image.c file was fixed, and the problems are first reproduced here. Later I tested without the (Pico) changes of the previous commit and there was no difference.
I take a look at the 0.128u7 commit. The MAME and MESS files related to the Genesis driver seems only adapted to the new core changes.
What seems strange to a non MESS/MAME developer like me are tags apparently unused, like the following part of src/mess/drivers/genesis.c:
The only reference to "soundcpu" (via a memory_region() call) I found was in src/mame/drivers/genesis.c, at READ8_HANDLER (genesis_z80_bank_r) which is only referenced at a disabled (with #if 0 .. #endif) address mapping in the same file.