No visible levels in Audio Mixer FCP 7

A strange thing happens once in a while in FCP 7.0.3 on my Mac Pro (running Lion Version 10.7.4): When I display the Audio Mixer window, the levels for the individual tracks become all of a sudden invisible, although they were there where I started editing and mixing. But then, no more green bars going up and down as I play the time line, except sometimes between two points on the time line that don't seem to have any markers or specific characteristics associated with those in and out points. Very mysterious. Tried to trash my preferences, did all the updates, rebooted the computer. If I load another sequence, then the levels are visible again. What could switch these levels off and on? This phenomenon doesn't affect the Master level that is visible all the time. Any suggestions?