PWM and audio noise

[keithv]

I'm certainly no expert but can you show the schematic and PCB layout? It might give a better sense of what you're doing.
I mean, are you dimming a LED or running an 500A TIG inverter with that PWM signal....

Try finding out where the noise is comming in. Is it on the power rail(s) or on the signal lines (or both)?
Add more decoupling/filtering on the power rails of the OP-amps or whatever it is that's handling the audio signal. Think carefully about how you're routing your PCB.

It seems to be in the ground. Even so much as sharing the ground with shielded coax where the signal isn't even going anywhere near the MCU still has noise.