Check the -12 volt supply to the motherboard first. The audio out on Amigas is generated by chips called "Op-Amps" (Operational Amplifiers). These particular ones require both +12 and -12 in order to produce waveforms needed to generate sound for you. If you only have +12 available to the chips, only the positive side of each waveform can be generated. Thus, it produces crummy sound as the bottom of each waveform is "clipped" off at zero potential. That could even cause damage to speakers if played loud enough.
