For the most part the engine behind Music Collection ignores the meta-tags when importing mp3s. For greater clarity the behavior from the USB is it plays songs alphabetically when the selection criteria does not include and album or a playlist. In otherwords it only plays the songs "in order" when playing an album (if the track meta-tag is in the file) or a playlist. Unforetunately I don't have access to my car to confirm but I believe (some please correct me if I'm wrong) the Music Collection only plays songs in the correct order when: playing from playlist; and by album order recorded directly off of CD (this would be the order in which reads the songs off of the CD). In the case of importing from USB it ignores the meta-tags and imports the songs in the dierctory order (which probably defaults to alphabetical). To get around this you must creat a playlist for each album with the tracks in the correct order and then do your playback via the playlists.
Personally I gave up on using the Music Collection as it doesn't have the album art, etc. and simply manage a 32GB USB that I keep in the console.
2011 535i xDrive, Dark Graphite Metallic, Veneto Beige Dakota Leather, Technology Package, M Sport Package, Executive Package delivered Christmas Eve '10, Mods - quad tips, 20% tint, JB4.