I guess it could be a loose connection. Have you tried a PRAM/NVRAM reset? That might fix the issue.
A few weeks ago, my Beige G3 started having very unbalanced audio. I thought the L output of the speaker had died... but then I listened and I could hear it very faintly. Checked connections, everything was normal... so I replaced it with another Beige speaker, hoping it was a speaker issue and not a problem with the logic board.
Turns out neither were problematic... I had it set way to the right in the Sound Control Panel. Put back the old speakers since apparently they made different versions of em and my old one was better.
So yeah, have you checked the Sound Control Panel? Had to select "Built-in audio controller- internal speaker" and then fix the balance.
Hope some of this helps.