I was recently working on a friend's SE/30 that had the exact same problem. The HD was working, then suddenly it wasn't, and neither was external SCSI. Booting from floppy was fine. His sound was also very quiet, even at the loudest volume setting.

In his case, we decided that some of the board traces around the SCSI chip had been damaged by capacitor leakage. This may be more subtle-looking than you were expecting. I know I was expecting "leakage" to be some obvious-looking pool of battery acid, but in fact it's just a hard-to-notice area that looks a little grainy and reflects light differently than other areas.