First tell us about Project and its Preset, and then give complete details on your Assests. Also, please let us know what version of PE you are using.
It is most likely an issue with your particular Assets and also your Project Preset.
Also if Sceneline mode is giving an issue go into Timeline mode and see if you can see what is happening. After you have used Premiere Elements for a while you will never use Sceneline view. Timeline view is much more flexible.
As Hunt says it could be your media... are you using 5.1 surround sound clips?