I just stumbled across a link for a project for an
X10 Volume Control. It looks like it has pages from a book called
Home Automation Hacker's Bible Book 2. It works similarly to a photo transistor in that it uses a lamp module to control a night light bulb in the same box as two CdS photocells which are each wired up to a speakers. Since the CdS photocells' resistance changes with light, the volume changes with the brightness of the light. This approach is safe since there is complete isolation between the AC power and the speakers.
I thought it was interesting and figured someone else here might be interested.
