Audio track keeps getting deleted
Very strange behaviour that I can't figure out. I am editing a music video, so I have one music track. I lay that on the timeline and then I want to add picture. The problem is, after I add video and then trim it by cutting and deleting a bit, the entire music track gets deleted too. Doesn't matter where the picture is on the timeline. There are clips before it, so why is the music track being deleted? Is there a way to lock the music track so that I can mess with the picture without it disappearing for no reason that I can figure out? Here is a screen grab of what I have so far. If I delete the highlighted clip, the whole audio track above gets deleted too.
Imac 27" Intel i7 and 15" Intel 2.5 i5 Macbook Pro, Mac OS X (10.6.4)