Music missing from storage management
Make sure that there is no spinning wheel on top of the "Manage Storage" settings for if there is such a wheel, it indicates the loading of extra elements i.e. Music, Photos, etc. If you have lot's of Music i.e. above 4 or 5 GB, it could take time for the "Manage Storage" settings to bring the Music element on top. 

As a last resort, you can simply sync an empty iTunes library with your iPhone and iTunes would do what it does best - it will wipe out the iPhone music library in no time. If you wonder how to open an empty iTunes library on your computer, see this: https://support.apple.com/en-us/HT201596