imac DV G3 (PPC) speakers suddenly distorted sound output

At higher volumes you can even see the speaker cones vibrating.

That is normal. If they don't move, they don't work.

However, it could be that someone else overloaded the speakers and caused them to fail. I think the first step is to see if the speakers have been damaged. Use a pair of external speakers or headphones and test the sound.

Failed speakers in G3 iMacs are not a wide probelm like the one you mentioned, or bad capacitors in eMacs. Generally, the failure mode is the passive cooling (read, "no cooling") system baking the PAV board until some part of it fails.

Just wondering if you ever figured this out? I just had the same thing happen. Had the old iMac DV laying around and decided to hook up a usb midi keyboard to it so my daughter could learn some piano. I tested it out and played with it for a day or two and it was fine. Then suddenly the sound output from the speakers became completely distorted. The piano couldn't really play loud enough to do any damage, so I don't believe its the actual speakers, but more like the circuitry that's driving them.

Exact same thing happened here. Sage 450 MHz iMac G3. Pulled it out to let the kids play some old educational games on it and the sound is suddenly so distorted it's unusable. If anyone knows of a fix I'd love to hear it. Headphone sound is ok for me. It's only the sound from the speakers.

After some more research I found the problem I was having - the speaker foam had dry-rot and it was causing the distorted sound output.

I fixed the problem by buying some laptop speakers from Amazon for about $12 (4 speakers included, I only needed two) and replacing the speakers, using all existing wiring. The result is decent. The sound quality is not what it was with the original Harmon Kardon speakers, but it's adequate for letting the kids play some classic games.

The speakers I bought were 36mm 2w version, which is actually too large. There was a 30mm size available (with I think is the correct size), but they were only 1w speakers and I worried the sound quality might be pitiful. I couldn't find any info on the original specs of the HK speakers. With a little surgery on the speaker case, I shoved the 36mm speakers in and they've been doing well so far.

I may at some point see if I can find some higher quality 30mm speakers, but at least I have working sound on the iMac again.

Im my research, foam rot is pretty common in these. I'd suspect that before any circuitry problem. It's easy to check if you take the bottom case off.