I believe that SG has targeted your issue. That 5.1 channel audio is the culprit if you do not set the correct project preset in the new project dialog as you open the program.
But, I would add the following....
When you record AVCHD format from a camcorder, you could be setting for what is called FULL AVCHD, that is 1920 x 1080, or NON FULL AVCHD, that is 1440 x 1080 which becomes 1920 x 1080. So, check your camcorder settings to see which one.
If your camcorder recording = FULL AVCHD and you have 5.1 channel audio, then the project preset (new project dialog) should = NTSC AVCHD FULL HD1080i30 5.1 channel.
If your camcorder recording = NON FULL AVCHD and you have 5.1 channel audio, then the project preset (new project dialog) should = NTSC AVCHD HD 1080i30 5.1 channel.
If you are in a PAL area, rather NTSC, then you would use the PAL counterparts.
Another option that you might have would be to record the Audio in stereo, instead of DD 5.1 SS.
As PrE cannot Export DD 5.1 SS Audio, or include it in a DVD, you will have a down-mix to stereo anyway.
I'd suggest that you test both settings (if you can change to stereo), as many have found that the 5.1 recording from the camera's mics. picks up a lot of unwanted signals, and stereo will narrow this down quite a bit. The choice is yours and Steve and ATR have furnished the answers for the DD 5.1 SS onto the Timeline.