I have a complicated prop I've built based on an arduino Nano, which uses an adafruit LED backpack that drives a LED bar graph as well as a 2 digit 7 segment display. The prop also uses a DFMini MP3 player to play sound effects.
When the LEDs are hooked up and lit, it causes an audible hum on the speakers. As the LEDs flash, the hum pulses - it for sure has something to do with the frequency of the LED driver affecting the speakers. What I don't know is how to fix that. Everything uses a common ground - it has to the way the wiring on this thing is done. Is there something I can put inline with the speakers that will reduce that noise on the line?
The noise is probably coming through the power supply. You can try a different power supply (or "bigger" battery), or maybe a capacitor (1000uF or higher) across the power supply/battery, or maybe separate power supplies (or batteries).
You can put a seriese inductor in the supply line to the various units to isolate the power supplies more. You can employ star ground wiring where the all the grounds meet at one point. Finally you can lower the input impedance of the audio amplifier by putting a resistor from input to ground, about 3K3. It will reduce the signal but it will reduce the noise more.
None of this is a guarantee, it is what happens when you mix digital and audio.