- do you have a 2 channel source loaded in the viewer?
- are you sure you're looking at the whole timeline?
make sure the blue scroll bar on the right hand side is all the way at the top of the audio tracks section.
- try a new timeline/sequnce.
there might be a corruption in your current one, although i've never heard of a corruption manifesting itself in this way.
you can copy paste into a new sequence if your old one is corrupt.
nick