I recently upgraded my slot loading iMac 400 from OS9 to OSX 2.8 and now the sound no longer works. When I play a CD for example, everything about I Tunes looks normal but no sound. The volume is turned up and the mute is off. When I open the sound preferences panel, I can choose warning tones, etc. but no sound comes out. There is nothing plugged into any of the external sound jacks. What can I try next? Don

I just upgraded my old iMac DV 450 mhz to 10.4 and the sound is garbled...still working on that, it's my spare. I'm thinking that it's either an internal connection or some piece of software that is redirecting the sound to USB. Can you boot from the install DVD?...try increasing the sound there. Does it have sound when you turn it on (the Mac startup chime)? IF that has no sound then there is something internally wrong with your Mac. iMacs are not and probably will never be easy to just jump in and check connections. Older iMacs like the ones we have a VERY difficult to pop open and check. If you really want to pop it open there are plenty of documents out here on the net to show you how to do this. I hope these suggestions point you in the right direction.

Yes, I've got the startup chime so the sound is physically working, at least at startup. It seems to me that I read somewhere that this generation iMac shouldn't be upgraded beyond 10.3...

I just updated to 10.2.8 and installed all of the recommended software updates to go along with that version and the sound is working now. In fact it sounds better than ever, it's like I upgraded the speakers, I'm impressed. The update mentioned that it pertained to audio and it surely did. Thank you for your help.