Audio? LM324 opamps?
An LM324 opamp is never used for audio (except it can drive a beeper) because it has crossover distortion, has a high noise level and has a poor high frequency slew rate.

 

The challenge of distributing power to a large number of similar circuit boards is met very well in the large studio mixer consoles. So researching how those are done could provide very useful insights as to how to do it. I once acqired a used power supply of that variety, surplus. It was indeed very electrically quiet, and quite stable as well. What I saw was attention to iring and filter capacitors, which were larger than the minimum required. It was linear regulated, not a switcher. With adequate filtering a switcher supply can be quiet, but the extra filtration will add to the cost a bit. The 12 volts out from most computer power supplies is mostly to drive the motors in fans and disk drives, which can work very well with noisy power. And the negative 12 volts used to be mostly for the RS232 portions, and other communications. Whie the +12 volts is many amps capacity the negative 12 is seldom more than one amp. That may be a problem.

 

The datasheet for high PSRR voltage regulators should give you an excellent audio filter circuit for the switch mode PS.
You might end up with very low noise power supply by putting together a post filter regulator board.
Think about one board but the design will use parallel regulators. 4 regulators rated 5A each should handle 20 amps

 

Audio? LM324 opamps?
An LM324 opamp is never used for audio (except it can drive a beeper) because it has crossover distortion, has a high noise level and has a poor high frequency slew rate.

Hi, No LM324 I use as comparators.