First of all, I have no experience with Mac's.
However, correct me if i'm wrong, if your iMac is a PC version then you should be able to do the following.
OK here's what you have to do:
1.) If you have any important data, make backups of them.
2.) Re-install debian - make sure you have the latest version, and I suggest you get Debian Sarge because the installer will auto-probe your hardware and install the necessary drivers.
I'm not sure if you could install this on an iMac so try and find out if you can or else just do what you did earlier to install linux on your system.
3.) Be careful when you install linux and
NOTE down the root password you enter when the installer asks for it.
4.)Now see if the sound works, if it still doesn't try the things you said you tried to get sound working but failed since you didn't know what the root password was.
5.)Once you've done all of this, come back here and post your results!
vxc69
p.s: I'm suspicious about your signature "IM Evil" - this is not a joke right?