Sounds like the typical battery leakage syndrome... the leaked electrolyte shorts out various functions, takes out vias and even traces which messes up the overall sound. I helped a guy sort his out, all the defective traces were already jumpered etc., but he neglected to clean off the battery residue and that was enough to mess up the sound and gave the same symptoms you describe - went away soon as it was properly cleaned. Could be the same issues with your machine, first you have to check for leakage and cleanup if so, then start troubleshooting any damaged connections....