So, I have been working for a VERY long time now on a series called Game Thing...! which is made in LBP2, but with lots of post-editing in Final Cut. Every time a character opens their mouths is a separate shot, recorded and edited in, and the we re-dub over the lines so they sound better.

This used to work well, now it does not. It seems more like random mouth flapping, rather than as if it is making any attempt at all to work out how to look good. I thought perhaps changes we had made to the set had made the system prioritise accurate lip sync to a lower degree, so i deleted stuff that wasn't essential to just have what was in the shot. This improved nothing.

I did did get a new headset since last time, which you may think could be the best reason, but the last one was terrible and the sounds were unclear. This one is crisp and perfect. I also did some animation tests with it on the day I got it, and the results were perfect first time, all the time.

But now the game is just refusing to take into account how many syllables are in a sentence no matter how clear i speak. Any thoughts?

If the new mic is crisp and clear it could be too sensitive and picking up ambient sound from the room. The game then translates that mix of noise and voice into mouth movements rather than just the voice. That's my theory anyway. Try going into the audio device settings and lower the sensitivity or try the old mic to see if it's still a problem.