I have an iMac G3 500mhz Dalmation. It does not produce sound through the internal speakers. The headphone jacks on the front work perfectly.
No change with zapping the pram
No change with extensions off
Everytime I restart the Sound control panel reverts back to the Headphone choice
I installed a brand new PRAM battery since it was dead. That didn't help.
I have removed all covers and see no visual problems with the speakers, sound board or speaker cables.