Well, I've been looking at that first youtube clip again and I think maybe they sang along to a pre-recorded version that they'd 'taped earlier' (sounds like a cookery programme!)
I say this because the version does sound a bit rawer than the studio version and there's a bit of feedback.
But I still don't think they're playing their instruments when being filmed.At one point in the clip ( from 0.57 to 1.02) Paul plays his bass when there isn't any to be played, he then stops and looks slightly perplexed.
I've also noticed something weird about the Hey Jude film but I won't go into that or you may say I'm off topic.
