This is because you are using AVCHD 5.1 audio in a project that is not set up for 5.1 audio.
If you start a new project and select the AVCHD with 5.1 audio project preset, your video clips should load to Video 1.
I'm not sure how successful you'll be combining those different formats though. An MOV may go into an AVCHD project, but it may behave funny.
And Premiere Elements probably won't work with AVCHD Lite at all. The FAQs to the right of this forum tell you how to convert it, though, to an editable format.
I'm not saying you absolutely will have problems. But it's a tough mix to make work, so you may want test it out -- and even output a test of the three formats used together -- just to ensure it works before you get too deeply into your project.
Thanks for the quick reply.
I checked the project settings - it is setup for 5.1 audio. It seems that clips with different audio (mono, stereo) cannot be handled in video track with 5.1 audio. It seems that AVCHD lite is the most challenging format. I will check the link provided by you and other discussions regarding format conversion.
If your Project is set up for the AVCHD Lite (the Lite version has been known to cause issues) and 5.1, you should still be able to drag your other footage to another Video Track (VT) and let PE assign an appropriate Audio Track (AT) to that footage. If you're in the Timeline View, you'll basically be doing what was called "A-B Roll Editing," where your Video is on separate VT's. This was how it was done many versions ago. Other than having to scroll about with your VT's and AT's, it's not such a big deal, and actually will help one visualize things, like Handles on Clips, more easily.
For doing A-B Roll Editing, I do like the way that the interface in PrPro is set up. All VT's are together, above a center-line, and all AT's are below that center-line. It keeps things together better, with far less scrolling. I'm sure that the designers of PE thought that most users would Import/Capture the same spec. footage, from the same camera, so designed the interface to keep the Video and its Audio more in unison. When I go to PE from PrPro, I am always asking myself, "hey, where'd my VT3 go?" Well, the answer is scroll.
PS there are some fairly recent threads on AVCHD Lite and PE. You might get some useful info from those.