How do I rebuild the music library on my iphone?
Most probably the music library on your phone got confused.

Remove the music from your phone and re-add it by


either removing it manually (if you are managing manually)
or unchecking music syncing in iTunes


Then sync once to clear the iPhone music library, add your music again and re-sync to relead.

If that doesn't work, backup your phone, restore to defaults, restore the backup, sync the phone. That will completely clear any cached data on the phone and allow it to sync "fresh" to iTunes.