I know I'm a bit late to the party on this, but I'm hip deep in a video project that requires fairly accurate lip sync. And seeing how long it's been since this topic was addressed, I'm guessing that a decent solution has not been suggested.
The ONLY time I've ever seen accurate lip sync in Second Life is from videos by the creator of the Black Dragon viewer:
From what I can tell, his lip sync is nearly dead on. I'm not sure if it's the Black Dragon viewer itself, the settings in the viewer, the computer he's using, internet connection or just a really powerful graphics card. I've tried reaching out to him, but he has yet to respond.
Now, if you have After Effects, one thing you might try doing is using Auto Lip Sync from mamoworld.com. It's a bit on the...goofy side, but it works. But then again, lip sync for "Robot Chicken" is pretty cheesy, but doesn't take away from the show.