The majority of the posts I see in this forum seem to be coming from guys with Windows systems. (Of course I may be blind). If you were on a Windows machine I could probably tell some things to try, but like a lot of people here, I don't have a clue about iMac stuff. You might want to try posting this question in a forum specifically for iMac problems.
However, you also might want to look at the sound card's mixer. I would assume that if it doesn't have a software mixer of its own then iMac probably has one similar to the Windows mixer. Anyhow, I would check the mixer to make sure all the proper outputs (especially wave and midi) are still checked and the faders full up. Also, look to see if the soundcard has any virtual outs on it, and see if everything's not getting routed to the wrong outputs. For instance, my MIA card's mixer gives me 8 virtual outs. If the main outputs I use going out of my soundcard into my playback hardware are outputs 1&2 but my card has those outputs muted and has routed most of the sounds to outputs 3&4 and their fader is turned down (or muted also) then I wouldn't hear anything on playback. Maybe installing CS4 messed up the settings.
Also, look at the file "associations" in iTunes and make sure it's still set to play the same file types (wave, midi, mp3 etc) that it had before.
Does iMac have something similar to Windows "Device Manager"? You might try looking at that and checking the properties of the soundcard to see if anything looks like it needs to be changed.