Songs missing on iOS 8 after sync
I had this problem and was able to resolve it by following the clues here:
http://robservatory.com/a-nasty-little-itunesios-bug-may-be-causing-media-sync-issues/

Apparently it's a bug triggered by duplicates in the iTunes Library.  In my case, I had a couple of duplicates that I wasn't aware of from several years ago.

I used the "Dupin" utility to find the duplicates.  I eliminated the extra copies.  I then turned off "Sync Music" in iTunes and did a sync.  There was still some remnant music, so I went into Settings on the iPhone, then General > Usage > Manage Storage > Music.  Click Edit and delete All Songs.  The Settings app will probably crash but the music will be gone finally.  Verify this in the Music app.  Then turn on "Sync Music" in iTunes and resync.