If you aren't going to power a PC with the supply as well (just the amp & 5v items), you may want to convert it totally like in this article:
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-SupplyI did this with my first bench-supply conversion (to run a 12v battery charger for my airplanes, works great, and I can connect any 12v/5v item to it with the adapters I made). The Corsair I have I didn't do as much of a radical mod (I just took a few power adapters and trimmed them back for the resistor part of the mod, not needed if you have something 5v plugged into the supply, and ran a jumper between pins 15 and 16 (green and black) on the main power supply header that goes to the motherboard. I'm working on setting that up switched from the other PC supply so it'll kick on when the PC comes on, and not run all the time as it does temporarily now, just need to pick up a relay I can use with it.)
For the initial tests, I used a old hard drive as a load for the supply, and used the HD adapters I had laying around for powering the amp & flipbook setup. Once I had it working, I cleaned up the wiring a bit (didn't remove anything yet, but that's on the to-do list eventually), and used several of the drive connectors to give a bit more wire to the 12v amp's lines. One other thing you'll need to do is jumper from the 12v input to the cut-on circuit for the amplifier (any size wire will do, all it's there for is to turn on the amp, could also use that with a relay to cut it on and off via a serial control, see the carmp3.com site for info on that).
**EDIT** I just checked the pinout on the main power supply connector, and shorting 15-16 is for a 24-pin header, the older 20-pin header you short 14 & 15 (still a black and a green pin on the standard wire colors though). Just thought I'd mention it in case anybody was using a slightly older supply..