Did you try replacing the internal battery with a fresh one? A low battery can cause strange behaviour (including corrupted sounds) in these units.
Other things to check for are interference (may be something as simple as being connected to the same power outlet as your PC), but in some cases the only solution may be moving to a different room or even house (if it's interference from a cell phone tower, for example).
Also check whether it's not just the volume pot that's turned wide open - because if it's that, and by accident you are able to make it work, the very loud sound might damage something. You did try playing a MIDI file, right? Speakers left/right connected properly? MIDI switch on the back of the unit set properly? (just checking the obvious)
I removed the battery alltogether, I've read some posts that said it didn't affect games, and can even be beneficial, as the unit resets itself after each power on. I did meassure the voltage of the battery, and I got 3V. But I'll try a new battery.
I didn't even think that the plug could be an issue, I'll try that as well. One thing to note is, the unit is from Japan, where the voltage is 100V. I live in Spain and we have 240V here, I'm using a transformer that outputs 110V (actually 116V when meassured with a voltimeter), could that be an issue? The PSU was one of the first things I thought of, but I think that may be difficult to find a replacement. It has two 12V rails and two 5V rails.
I can turn the voume up and down with the knob.
I tried connecting it to 2 different PCs, one directly to a AWE32 through the joystick port and with a MIDI interface card. The unit doesn't react. I don't get anything on the screen. It only says "Piano" on the top.
I tried connecting speakers from the RCA output as well as the headphone jack in the front.
The MIDI switch is in the MIDI position, I tried the others as well just in case, with the same results.
The thing is, even when there's no MIDI input connection, I still get the same static sound.