This article is from the Macintosh hardware FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
When the metal fingers that connect the IIsi speakers and the
motherboard get rusty, sound can blank out completely. This may not
occur until the IIsi has been running for several hours. As a quick
fix turn the speaker volume all the way up and then down again. This
sends a small spike of electricity across the contacts and should
temporarily dislodge the rust. If this doesn't work a whack on the
side of the Mac may also clear the contacts enough to restore sound.
For a permanent and easy fix plug a pair of external speakers into
the sound out port. I've heard at least a dozen different
suggestions for permanent fixes to the internal speakers. The only
thing all suggestions have in common is taking the speaker
subassembly out and putting it back in again. When you're having
trouble with electronics, there's just no substitute for pulling
everything apart and putting it back together again, and that's all
you really need to do to fix the sound on your IIsi. The speaker
assembly is hidden under the hard drive so take that out first. Once
you've removed the hard drive, spread the four tabs that hold the
speaker assembly in place and slide it out. Then clean the contacts
with isopropyl alcohol. Even rubbing them hard with a paper towel or
lightly with sand paper should dislodge enough rust to fix them. You
can even coat the contacts with an anti-oxidant like DeoxIt or
PreservIt to prevent future problems. You should be able to find
some such compound at any electronics supply store.