You can also press the diagnostics button on the sound card and it will play some test sounds. That way, you are not relying on a signal from the driver board.
Press the diagnostic button and while that is running, adjust the balance pot on the voice board. You can also adjust the main volume in the cab.
The sound board is no different than the CPU board in that it can suffer from the typical cold solder joints, bad capacitors and bad IC sockets. Since you replaced the caps already, reflow the solder on the header pins.
Then its a matter of using a logic probe to see if the CPU is running. If its not, I would replace the ROM socket, try again. Then the CPU socket.