Saw this issue discussed in earlier threads, but didn’t see a ‘how to fix’ solution.  I’ve just had this problem.  I’d previously transferred all the CD albums to desktop in FLAC format to match CD quality; no problem.  But when I transferred the music files to a Flash drive to download to WMP on a laptop all of the albums were missing track 1.  Spent ages trying to resolve the issue and eventually tripped over a remedy.  I noticed that all the missing track 1’s were listed in a ‘Unknown Album’ folder; but I couldn’t see a way of reinstating the tracks.  I tripped over the solution when using the WMP ‘Find Album Info’.  Beneath that dropdown menu is ‘Update album info’; click on this and hey presto track 1 returns.  Incidentally this problem didn’t occur with mp3; the issue seems to be with mp4.  Hope this helps someone.