If you do not have AVCHD with 5.1 channel sound but rather MPEG2 with 5.1 channel sound, your workaround is to work in the Timeline View, instead of the Sceneline View. If you are recording MPEG2 and can change the 5.1 channel sound to Stereo in the camcorder that is great, but in most of these cases the camcorder manufacturer does not let you do that and you are stuck with the 5.1 channel sound.
If you need more details, please let me know and I can supply you with the background information.
If you have AVCHD with 5.1 channel sound and set the correct project preset as referenced in the FAQ, you will end up with your video on Video Track 1 and your audio on Audio Tract 1 (Timeline) and video clips additions in the appropriate scenes in the Filmstrip (Sceneline).
Steve and Tony have nailed what is necessary to get your Assets to "behave" properly - the Project Preset for the DD 5.1 SS Audio.
I would also suggest that you take a long look at the Timeline View Mode. That is where most video editing will be done. It will also allow you to see what is happening, regarding the Assets. I cannot recommend looking closely at Timeline View Mode too strongly. You will not regret doing so.
Thanks! Problem resolved!