Whoa boy!
We were talking about the cdrom. Why did you do a whole slew of other stuff without addressing whether the cdrom audio was getting thru to the 2496 first? Technically, once you enable the digital audio checkbox on the cdrom in device manager, you could remove the cd audio cable (the thin one) and the audigy should still have cd audio because the audio data is now flowing thru the IDE bus.
When you pulled out the audigy, did you uninstall it in the Device Manager first? If not, you could leave latent device drivers in your registry.
It's ok to disable the onboard audio if you're going to use the 2496 as your only audio device; you don't have to enable it when you install the maudio. I don't own a 2496 personally, but I've seen others who have no problems installing it in their systems.