I have the same issue. The workaround Ive found is this (is brutal though)
- Album by album select every song manually so your phone recognises the song wont play and it greys out
- In iTunes, deselect the album and reselect again
- Sync
- Songs now play on your phone.
Is painstakingly long but it works. Ive been going for 5 hours at work and are only up to F so will take a while, but it has fixed it. Ive got 15000 songs on my phone so will be at it for a while.
Any easier options please advise as this way is killing me!