Quoted from frunch:

I'd definitely start with connectors as far as the sound issue, both on the sound board and mpu board. As was said earlier, if reseating the connectors fixes the problem, don't wait--replace the connector pins. And even if replacing/re-flowing the connectors doesn't fix the issues, at very least you can rule them out and proceed from there. I'm really surprised how often the problems on these games comes down to connectors.
For that matter, i think it's also worthwhile to take a closer look at the rectifier board. Any burned up connectors there? I always replace the male header pins and female connectors on the rectifier board when i get a game of this vintage. Voltage problems (often resulting from old, worn-out connectors) starting at the rectifier board can lead you down all kinds of diagnostic dead-ends elsewhere. So make certain all connectors are good there, and that the fuses are properly rated and the fuse clips are good and tight.
One last thing: the big capacitor on the power supply board--does it look original? Many had a big metallic blue one. If so, you'll definitely need to replace it to ensure good 5v and 12v supplies.
Good luck! Xenon is a great game, can be very difficult!