It looks like I was wrong. For one thing, l3codecx.ax is another Microsoft file. I reinstalled ffdshow but left mp3 files disabled in its audio config, and I still have the stutter problem even though ffdshow doesn't show up in the test render. Just the fact that another codec is installed is causing the stutter. I'm inclined to believe EMC is the problem, not codecs.
I can confirm it well. Without CCCP installed (and nothing else) EMC works perfect and does not cause any stuttering. With CCCP installed I get alot of stuttering, regardless of which setting is on or off.
The strange thing is I never had that much problems with using EMC and my installed codecs, only after upgrading Vista to SP2 did the stuttering begin. Atleast, that's what I suspect. You see, at the time I upgraded to SP2 I wasn't playing or modding Oblivion so I didn't notice it right away. But when I came back to test a mod a bit I suddenly had great stuttering (and even more when in dialogue) and I couldn't think of anything else I might have changed. :shrug:
Although I would have loved to continue to use EMC, with my current play/mod ratio I'd rather have good performance for those brief moments I run the game than the benefits of immersive music handling.
Still, EMC is a great mod (or resource?) and I've used it for a long time without problems.
-kyoma