Hmm are you privy to the contract details then guitar?
Thats not what I came to say really. I was interested to see how long we were going to confuse the very skilled art of lipsynching with on stage miming. As an ex Broadcasting film dubbing mixer I know the difference. Luckily my high horse is not needed here because guest:guest at 1141 just above showed the detail that a pro enjoys and most observers only notice as an uneasy feeling of unreality.
Lipsynching or post dubbing has been around since the early talking movies. The picture was obviously more important at the time of shooting and is much more controllable. Sound is is a wonderfully complex sensation. The beauty of which is completely cobbled by overhead planes, distant motorways and car horns etc., especially in a period drama or a scene on a desert Island. It is the dubbing mixers art to restore the background required by the writers vision, and to re-record the actors voices in a controlled studio environment carefully adding the correct acoustic.

Oh I'll shut up now I'm waxing poetic, always a bad sign.
Any way Miming on stage is NOT lipsynching. I expect it's yet another accepted word change by common useage. Still, only a few years now to my free bus pass!!

Andy