Why my head unit functions but I get no sound?

make sure your speakers work through Aux ports, or BT connections if you can. if they dont, then you will have a failing speaker set, and it wont be the Hardware. be sure to test Discs vs Radio Vs Aux. Make sure all are failing, or just one.

if your audio has network capabilities, running a firmware update may be required.

i would start with a full tear down. Remove the device, disconnect all cables, and reinstall the cables. could be a loose connection in the back. (if you occasionally get sound, that means the hardware works, and if the cables are loose, it would intermittantly send the signal to the speakers.

if you have access to a second Dash audio system, i would plug it in, and see if it fixes your issue. This will verify if it is something in either the car, or the system.

if these dont work, it gets a bit more tricky. Make sure the connections inside are proper. check the continuity of the cables with your multimeter, and as many 2pin components as you can. if you get a component that doesnt pass a 30 second continuity test, replace it. Anything further, testing would require full schematics of the specific audio system, and comparing present, and expected voltages.