I'll assume you meant G3. We've been discussing the sound problem around here for awhile and it seems to happen now and then on G3 systems. There seems to be some sort of bug in the davbus sound drivers for MacOS X when it comes to G3's. So far there is no fix in sight except for iMacs and G3 upgrades.
With an iMac you can try these: enable and then disable mute in the sound control panel; unplug exterior speakers; use the headphone jack.