On power on, so you get one or more bongs?

If you're not getting a tone on bootup, then either:
(1) The Audio board isn't getting power. If this is the case, you will get an test message saying 'Audio Interface Error', or
(2) The speakers are disconnected, or
(3) The amp on the board is blown.

It's unplugged.
Check all connections to the audio board. Especially, since you have custom DMD, any Z-connectors. One is probably loose or missing.

I don't know how the Color or LCDs work, so I personally can't help there. The +5v for the Audio board is J502, on the left edge of the board, under the ribbon cable.

Those fuses will not cause the Sound Interface Error message, as those fuses run the amps. Dead amps, working logic wont cause the error message.

Again,
For the record - if he IS getting the 'Sound Board Interface Error' in Test Report AND no bong, it is NOT speakers or amp, or amp circuitry (fuses on sound board).

Why? Because the 'Sound Board Interface Error' means that the game got no response from the Audio board on the ribbon cable on bootup. A bad ribbon cable CAN cause this error - but then you would still get the bong from the audio board on power up. Other causes of this error can be another board on the ribbon cable acting up, or the audio board isn't booting. In the last scenario, you would not get a bong either.

Of course, there COULD be two things wrong - speakers unplugged and bad ribbon cable. But I am assuming that right now, it's a simple power issue to the audio board.

Then the board logic is fine - it's Amp, fuse, or speaker connection issue, yes.