I have a directory of .mp3 files from a CD, and Rhythmbox imported them and plays them without problem.
But the the first six tracks have "Unknown" for the information fields:
How can it get half the album details correct, and not the first half?
That's odd but perhaps it's a "bad" CD.
You can check if the songs actually contain metadata by opening them with these. If you can't see any metadata then I guess Rhythmbox isn't the one to blame here. Try EasyTAG, from what I hear it's pretty good.