The earliest model iMac G4 (700/800MHz) can run OS9, I think, but a lot of the later G4s require either a machine-specific version or 9.1+ to run, bleah. Classic mode in OSX is done by having an OS9 System folder on your hard drive and pointing OSX to it in the Classc control panel; set it to activate silently on boot and you can use a pretty good number of classic Mac OS programs transparently within OSX.
The strange plug is Apple's digital audio connector (is this S/PDIF? I don't know.) Anyway, yeah, no audio in, unless you can find a USB solution :/
