Hey, guys. I am very sorry to hear so many people have the same issue and by reading most of this thread, it can probably be deducted that this is a bug within either the new version of the iTunes or current versions of iOS7.
I have managed to solve this twice now, with three different albums that I wanted to add to my music library and to my iPhone 4S 64GB.
First, I unsynced the music as was suggested by one of the repliers, but honestly, the prosess of wiping out your entire library is quite drastic and I have to say that it absolutely should not come to this. However, the first time I tried it, it worked, creating a reptilian music library on one of my drives.
The second option is much less bizarre. I added the albums to my iTunes library and synced them to the iPhone, only to get all the tracks ghosted (gray with a red stop-like sign, unplayable). Then I deleted them and restarted (not wiped out, but simply restarted by holding the block button and the home button for a few seconds) the device and restarted the iTunes using the Task Manager (my laptop runs on Windows 7, but it should make much difference here, I suppose).
I deleted the songs from the iTunes and added and synched them again, album by album (superstitiously trying not to overwhelm the iPhone with too many problematic syncs at once). It worked with the first album, but with the second, it didn't. That's when I decided to just leave the music in the iTunes without removing it and adding it all over. So I simply restarted the devices and iTunes once more and, upon connecting the iPhone to the laptop and opening iTunes once again, I synced the music that was already there - the two remaining albums - and it worked, they simply unghosted. I suggest that you close all the tabs before syncing and opening them when the sync is complete.
Also you can chant a bit and flail around with a tambourine - I don't know if it helps, but lately performing the simplest tasks through iTunes seems to require many primitive and bizarre rituals that should have become outdated the second phones became "smart".
Good luck! I really hope Apple fixes this, but remember that most of Apple support are operators and everyman techs, and changes in iTunes and iOS are authorized and made by the big guys. Apparently the same ones who may think 100,000 hits on an issue thread and posts from perfectly reasonable clients of more than a decade about considering the competition software and devices are unworthy of immediate reaction.