Check the motherboard to see if any capacitors have leaked. If they have then there will be some electrolyte spilled on the motherboard around the capacitors which may be shorting out components. This should be cleaned off somehow - I find the best way is to use Q-tips and isopropanol, but most cleaning solvents should work fine.

When my IIcx had startup and restarting issues I cleaned the motherboard in this way and it solved the problems entirely. It may well be that leaking capacitors are what are causing the sound issues with your IIci.