That means that the sound amps are low power.
You should have used a 5volt supply for everything (no buck converters).
And maybe... a 5 to 12volt boost converter for the unspecified amps.
But 5volt class-D should be better there, so a 5volt-only setup.
A 12volt supply is just plain silly.
Was the built-in class-D amplifier on the Mp3 module not powerful enough?
Leo..