Turn up the volume pot to as high as you can stand it.

Press the sound test button on the CPU (SW1). What do you hear?
-report back here
Now, reboot the game and go to the actual menu driven sound tests. Hear anything? Even faintly? Possibly very faint blips and bleeps.
-report back here
Also, I would definitely check the Ohms of all the speakers. (The Ohms rating should be printed on each speaker). The hum could point to a bad speaker. If you find it, remove it to help your troubleshooting along.