Basically, I was having problem with my phone so I took it to an Apple store and they restored it.
After restoring the back up onto the phone, over half of my music has synced onto the iPhone as 'other' data and does not appear in the music app.
Sounds like you have some corrupted music files.
It can't be. They all play fine on my laptop, and before taking it to the Apple store, they all synced onto the phone fine.
Retrieving data ...