The fundamental problem is that supergun arcade game boards provide amplified audio by default.
Consolized arcade boards have to have an attenuation circuit to "normalize" audio to linear level.

There are countless supergun conversions so it's pretty much impossible for to say which one is good.

Given most supergun conversion developers don't properly consider audio, they simply use two amplified wires on the SCART connector or RCA and provide no explanation on that.
This is why many users fry their OSSC's and the gcomp/ gscart switches with them.