I'm completely baffled by the percussion section on this thing. I can't see any real reason for the ridiculous number of resistors on the percussion circuit half of the board. You could create those sounds in a far simpler manner.
Theres some halfway decent glitches and the shorts that assign the speech phrases to the keys, up on the connector for the black blob chip. As long as you stay away from the VCC pin 7 it should be alright.
I replaced the ceramic resonator on the black blob with an LTC1799 circuit. Ths controls the pitch of the speech and animal noises. The pitch of the normal keyboard sounds is controlled by another ceramic resonator on the biggest chip, but i didn't bother with this one as it also controls the tempo of the drum loops which i find really annoying.
Checkout the lcircuit layout on the sailormouth site
http://www.sailormouth.org/vtmmold.htmlHe seems to have gone to a ridiculous amount of effort on this thing and then found very little of any use, which seems to have been everyone elses experience as well.