Would it be beneficial to use a separate power supply for the amp?
If you could give me some advice on how to decouple my circuit (maybe a starting point for experimentation) would be most appreciative.
Best not to use a switch-mode supply if possible.
Decoupling: something like 2,200uF electrolytic, 10uF electrolytic and 0.1uF ceramic (some of these will already be on the board).
One glaring issue is feeding a 48kHz sampled audio signal into a 250kHz class-D amplifier, lots of heterodyne whistles to be expected - the class D clock should ideally be derived from the sample clock. I suspect a multipole low-pass filter is needed on the audio signal to eliminate the sample clock.
Also note the amp's input is differential so you must pass both audio signal and ground wires between modules for the audio as well as common supply ground.