I heard Mel say in an interview once that the scene early on in the movie where he meets Aunty Entity for the first time had to be entirely revoiced.
He said that, of course, in reality the whole thing was shot only a foot off the ground in a studio, rather than at the top of that tower, but that a bunch of birds had made their home somewhere nearby and they could do nothing to shut them up.
So they filmed the entire scene with the bird noises, and revoiced the whole thing later.
The interview was on the (night time) Mike Walsh Show at the time of Thunderdome's release.