A music theme level isn't the same thing as a level where the monsters are singing and clapping. The piranha plants have lip synced singing and clapping which makes most of the music. When a new plant enters the scene you can here the new voice starting, kill it and that voice dies.

For example, in rayman legends they have music and then spawn monsters with the instruments of the music, but those monsters has nothing to do with the music since the music is a static track, killing them or them spawning doesn't change anything.