Assuming you've imported everything from CD, you should still be able to get the track names from CDDB. Simply sort everything by date added, select each "batch" of songs that represents a CD and then select "Get CD track names" from the Advanced menu.
The "batch" part is important because you must have the full compliment of tracks from each disk for this to work. It should be relatively easy to do so long as the first track is at the top. My understanding of CDDB is that it bases everything on the song length and count to make up a CD so it is very important to make sure that when you select a batch, it looks the same as if you were selecting it the first time you uploaded your CD, don't select any more nor any less.
Hopefully this'll work. Good luck. 🙂