Amiga 1200 sound issue

[Pat the Cat]

First, bad +12 and or -12V supplies are known to affect all Amiga's  sound. Paula uses them, and as well as being a digital sound chip, Paula  also handles the serial port Tx and Rx pins. So checking those pins are really giving  out those supply levels from the power pack are well worth it.

Second,  A1200s have a few capacitor changes, originally from the days of CBM.  This has been tested and tweaked over the years, but was originally  intended for compatibility of accelerator cards.

http://ianstedman.co.uk/Amiga/amiga_hacks/A1200_Mobo_fix/a1200_mobo_fix.html

So  that could indeed be where the problem really started. It would also partly explain why CBM never released an "official" A1200 accelerator.