Hi, I got the same problem. Source is a .mov file (plays normally on Quicktime), filmed with a photo camera Panasonic DMZ-L5.
I extracted de audio using Adobe Media Encoder to PCM (Wav); surprisingly, if I import only the wav file it also shows a muted audio track when dragged to the composition.
It is the very first time this happens to me with this camera, I never had problems before. Seems something related to drivers, but as far as I know Quicktime should be must than enough to handle .mov files.
My rig is a Intel i7 with 8GB of RAM, running Windows 7 Ultimate x64, fresh install so all programs are at last version / patch.
Walter