With respect to the G5, It might be time to see if using CHUD 3.5.2 will help. I know that oneboyarmy said his power supply chirped on his G5 when he engaged nap mode, but maybe you'll have better luck. On the 840av, if I remember correctly, the motherboard uses the normal two pin header that Apple used since the original Apple II. Could always try the speaker from your IIcx in there and see if it gives you the startup chime.

-J