I apologize in advance if you've already tried this or this isn't related, just trying to see if it'll help. I've had this happen to me before with my iPhone before and found that it was actually something wrong with iTunes. To fix the issue, I selected my phone in iTunes when connected, went to the Music tab, and selected the button "Selected playlists, artists, albums, and genres". From there it listed the items in my iTunes library, and I could check different items if I want them synced. I did notice that upon switching to the "selected items" button, my voice memos from my phone weren't checked. I'm not sure if that's a default setting or not, but I vaguely remember having the issue earlier on when I had imported some music from Windows Media Player. Hope this helps!