Audio problem: During lunch at school, Alvin tells an anecdote about Beyonce and all the girls laugh loudly, yet their mouths aren't moving.
Audio problem: Near the end of the movie, Alvin, Brittany, Jeanette and Ellanore are hanging tight to the helicopter. Jeanette jumps down to catch the remote. When she catches it you can hear "Way to go Jean!" Neither Alvin, Brittany, or Ellanore moves their mouth.
Chosen answer: Because they're more sensitive and weaker.