smps for audio
1st, ths PC PS has a number of fixed voltages and generally, only the 5V supply is a high power one.
2ndly, thre are +/-12V or +/-15V symetric supplies but they are low power supplies (low current)
3rd you must decide what is the power you want for your amplifier (grosso modo P=U*I) .Generally they use symetric last stages, that means you must use a symetric to ground PS and the conclusion is only the 12/15V outputs fit your need.
There are more thoughs, about efficiency and high frequency noise but 1st you must decide, based on paragraph 3 and 2 if the PS satisfies you and we ll maybe continue this discussion.