Determining Noise Filter Requirements
Figure out how much audio noise you can tolerate on the output, and work back through the PSRR of the amp and the parasitic paths to come up with a ripple spec on the power supply.

That's easy to say and not necessarily easy to do.  Simulation can help, but you may need a couple of iterations of PCB layout and test to dial it in.  Running a PSRR sweep on the audio path portion in the lab will be very helpful.