On our previous boat, the stereo went nuts when I keyed the HAM radio
on any band. I tried several fixes, but what finally worked was using coax for speaker leads.
Try hooking up the speakers with really short wiring
and see if it makes any difference.
If it does, you could use shielded wire.
I realize that was RF induction, but it might be worth a try.
BTW, it was a home type AC stereo system with CD changer etc.
We bought it because for the features vs the outlay, we could also buy another solar
We came out ahead powerwise by a long way, even with using the inverter