The OP says that the music directory on his iPhone has "disappeared" – but it was my impression that iPhones, like iPods before them, don't store music in a user-visible part of the file system.
You couldn't treat an iPod's music library as if it were a user-accessible directory on a USB flash drive. And the iPhone's Music app doesn't make the songs it accesses available in a Files app folder. I could be wrong, but I really doubt that turning on "Sync Library" is going to change that behavior.
I think there may be third-party music player apps that you can install for iPhones / iPads / iPod Touches – ones that play the music from whatever place Music hides it. This is similar to how there are third-party photo apps which have the ability to work with photos in the iPhone's {Photo Library / Camera Roll}.