Been thinking about this again the past couple days. It's interesting to me that
@XtraSmiley has seemingly the same issue on a Tengai running on a SH404 board, whereas the rest of us have had the problem on PS5 series (SH2 base, totally different system really).
I have Strikers 1945 on a SH404 and there is no problem like this. Rather than the sound routing on the PCB or associated circuit, I wonder if the root of the problem could be a shared chip/amp that is going bad on these? Caps are already ruled out, I know I've personally recapped mine and I believe
@ekorz has too.
On a side note if anyone wants to buy or trade for a Gunbird 2 with buzzy audio, I might know a guy.