MusicBrainz Picard does a reasonable job of recognising unknown music, using
AcoustID's of the sound itself. There are other applications that use AcoustID's as well, and the commercial ones are supposed to be better. Picard may struggle with classical music though.
XLD just looks up the free CDDB by the look of it, so if it can almost instantly recognise the music there must already be tags in the file. It doesn't look like it looks up metadata when you play a file, just when you convert a file. But I could be wrong in that. XLD also uses AccurateRip to check the RIP integrity. So I suspect there are some tags in the files already.
Have a read of
this post, and look at the images that show how to check what tags are in a file using MC. Specifically;
You can check what tags have been written to any media file from within MC. Just highlight the file, open the tag window, and click on the first line at the top of the tag Action Window, which shows the file type, duration, and size for audio files. See the first image. Then the tags stored in the file are shown in the Action Window. See second image.
Of course if there are tags in the files, MC should have picked them up during import. But if it didn't, and tags are in the files, you can use the Update Library From Tags function to get the information into the MC library.
