Please be careful. Unplug first when working on anything electrical. 110V AC can kill you!
Step 1: Acquire parts
--Pair of amplified computer speakers. A medium-to-large case would make things easier. Also, a standard, integrated power cord (non-wall-wart transformer) can free up a lot of space inside. I used a used set of Gateway Edison 2.0 speakers I bought from a used computer store for $5.
--An outlet, which I bought at a local hardware store for $0.59. You can decide if a cover is required for your application.
--2 Rectifiers. I bought mine from RadioShack. I decided to buy the 4A, 400V-capable units, to be on the safe side. They cost about $2.50/apiece.
--2 Solid State Relays. I followed Alan Parekh's example and used 2 Crydom D2W203F relays. I bought these from Mouser for $8.80/apiece. These are by no means the only units that can be used, but they work great.
--2 Potentiometers, to match the original that will be replaced. Mine was a 10k. These cost me $3/apiece at RadioShack.
--2 knobs to fit on the potentiometers. These cost me $3/pair at RadioShack.
--If your speakers have a wall-wart transformer, you will need a power cord to get the 110V AC inside the speaker cases. You can buy these from RadioShack, hardware stores, or simply cut one off of an old appliance you are no longer using. Again, please use caution and unplug anything before working on it.
-Small-gauge wire (I used 22ga stranded)
-AC-rated wire (or just cut a bit off your power cord, like I did)
-Some others may be required. I also used scissors, a cordless drill, side- and end-cutting pliers.