As the others have correctly mentioned, it's the
electrolytic capacitors leaking. It's not a fault with the design, more of a problem with the long term stability of the liquid electrolyte inside the capacitor. It doesn't just happen with Amigas.
My audio test utility is not pointless as it creates a fixed frequency, fixed amplitude signal in both channels, which is ideal for level comparison between amplifier stages and between left and right channels.
Listening to music output can give you an idea if things are working correctly, but you can't easily see or measure the distortion or do a level comparison between the channels.
Depending what components have been affected, one of the audio stages can self oscillate which will introduce signal distortion which can be difficult to pin point if you're just using music as a source.
And lastly, all my notes and measurements in the article mentioned previously are all based around the output of that program. So that provides many fixed point references which would be impossible to do otherwise.
I must have repaired more A4000 audio stages than I've had hot dinners, so you can say there's a bit of experience in there as well.
