Hi!
If you have multimeter, or any voltage measuring device which can scale to 0-12V, please measure voltages on U14 chip (TL084CN) on pins 1 and 7. Shouldn't be much more than 2,5 volts. Please look at this pinout diagram --->
http://circuits.datasheetdir.com/37/TL084-pinout.jpgI had 2 channels knocked out of my 500+ mobo, so only could hear whisperlike sound on one loudspeaker. I found out, it had 11volts on pin 1 (Right audio output pin), but only 2.x volts on pin 7 (Left audio output). So you should take a look at that chip also, it's amplifier chip on your Amiga.
These chips are easily obtainable, and cheap so you should be able to replace it if it's at fault.
Good luck.