Well, for audio bugs that actually matter, you can just listen to it, or if you want to get all sciencey, throw the output and input onto a scope and compare them, or send the input and output sample values into the analysis software of your choice.

Code isn't necessary to judge the quality of an audio system. You can pretty much treat it as a black box if you want.

The fact that no one has apparently even noticed this bug before says a lot.

The point is more that, yes, it’s technically incorrect. But if literally no human noticed or was affected in any way, who cares? That’s the only reason bugs matter anyway.