I had the same problem, it was indeed a matter of missing codecs for the open format files were imported. However I did not want to install ubuntu-restricted-extras since those included several plugins I'd rather not install right now as flash and java.
I solved this by sudo apt-get install gstreamer1.0 which installed a few libs and plugins.
Do you know which files were not imported? This could be an issue of missing codecs and a confusing error message (for example .wma instead of .mp3 or .ogg).
You can try to install additional codecs to play some non-free media formats by installing Restricted Extras via Terminal as follows:
sudo apt-get install ubuntu-restricted-extras
If that doesn't ...