Saw this sequence on TV when I was a teenager, never been able to find where it came from ever since.
It was a B&W movie, and there was a musical number in it, but I don't know if the movie itself was a musical. A woman is on stage singing a song, and to her stage right is a girl dancing. The dancer is slowly flanked by other women wearing sour-faced masks, while she herself is generally smiling as she performs. Eventually, at the song's peak, the masked dancers surround the girl and obscure her from view. When they disband and walk away, the girl is crumpled on the floor, and when she looks up, is now wearing an identical sour-faced mask as the others.
The only clues I can think of are that this TV station I saw it on only had films from a few studios, and played mostly stuff from a package of pre-1948 Paramount movies, but maybe a few RKO and WB titles too, but that's it. It's not, say, one of the classic MGM musicals - I've used the keywords of masks and dancers repeatedly and always get stuck on one of those bigger movies but they aren't it.