Why is it that during a party or dancing scene in movies or TV shows...?

Question

the directors don't play any music?

Answer 1

exactly what linav said, so you can hear the dialogue. What happens is they call for playback before they start shooting so the actors have a beat to dance to or at least bop to. Then they cut the music and everyone pantomimes their dancing action so the actors can say their lines and the boom can pick them up audibly. Then they put the music back in so that when you watch the finished product it looks like a party or a club or whatever they want it to be. Voila, movie magic :-)

Marianne

Answer 2

so you can hear the dialogue