Sound system? Are you trying to run a car stereo off a computer power supply? For that you don't need a computer power supply, you need a bench power supply which gives 13.8V DC and 10 to 75 Amps. Those are the ones people use to run demo sound system setups like the ones you see in auto studio. But those don't come cheap, even a 12 Amp cost little over $100 (US), 55 Amp $300+.
<font color=red>Got a silent setup, now I can hear myself thinking.... great silence</font color=red>