Buzzing, quality loss. Relay control system
Suspect the RFI of Raspberry operation, at very high frequencies and very fast edges, coming thru the coil-drive to the relays, is coupling to the Audio.

You may be hearing the program-execution RFI.

Put 0.01uF (10nF, 10,000pF) to GND, on each drive-signal to the relay PCB.
Which GND. Right at the Raspberry PCB. Beware the surge currents as the MCU charges and discharges the capacitors. The MCU may be damaged.

Thus ferrite beads into 0.001uF, to the relay coil, would be safer.