There may be a reset that could (perhaps) allow the speakers to be recognized;
such as PRAM/NVRAM (parameter random access memory.) Or PMU reset, (a
'power management' reset). First simply uses a 4-key combo on startup. And
second one requires a screwdriver to remove bottom/base plate from iMac G4
and under there, is a tiny button. Carefully. And see instructions in blue link(s).
~ Or maybe a restart in Safe mode could help?
Unless some damages are present in the plug end of speakers cable, or maybe
the port inside the iMac; in which case you still could try generic stereo speakers
and see if the headphone jack could work. My iMac G4s (800MHz + 1.25GHz) did.
{You could try the to reset/use the speakers both before turning on the iMac, or
even have the iMac unplugged before(?) Also the sound may be affected if the
tiny PRAM battery may have failed to hold power, so it may need replaced.}
The speakers may (or not) be correct, too. There had been a G4 Cube tower
with a connector similar to those the G4 iMac has; but not quite the same.
These port/cables probably would not interchange, though.
And then, one could use a powered set of speakers to utilize output
from the stereo headphone port; this automatically cancels internal
speaker sounds, except for an occasional restart/reset chime.
Good luck & happy trails! 🌞
