Music and ringtones won't sync (iOS 7, OSX 10.6.8)

My situation is a little... strange, partially because of how dated everything I have is.

I'm on an older model iMac that won't run any version of OSX beyond 10.6.8, the version it's running now, and an iPhone 4 on iOS 7.1.2. I've used iTunes 11.4 for syncing for a long time, but a while ago it just stopped transferring music and ringtones to the phone.

I've tried restoring the phone both from a backup and as new, but it hasn't changed anything - whenever a file is supposed to be transferred, it shows up greyed-out with the dotted circle in the phone's submenu in iTunes, and not at all on the phone.

Apps sync fine (with one exception, but that may be related to the app in question having been taken down), as does everything over iCloud; as far as I can tell the issue is specific to audio files.

What can I do to resolve this issue?