Audio problem: When Susan suspects someone might be in the house, the seduction music she has put on for Mike is drowned out by the score. When she accidentally hits Mike across the face, the score stops, but the music that was playing in the background has gone. The song in the background before now was music within the show, and it was loud enough to be heard around the house, so we should still be able to hear it.
Continuity mistake: When Bree is about to leave the hospital she picks her bag up with her left hand to put it on her right shoulder but when the camera angle changes she's put the bag on her left shoulder with her right arm.
Continuity mistake: When Bree is talking to Mike in hospital, the position of her roll neck jumper on her right shoulder changes constantly throughout the scene. In some shots it's rolled down, in some it is rolled up.
Answer: Kayla was sent away in episode 16 (The Gun Song) in season 4, after falsely accusing Lynette of child abuse. She now lives with her grandparents - probably Nora's parents.
I'm curious why we never see her again, why she never talked about, and why Tom never mentions visiting her.