If you want to set your favourite music as your iPhone's ringtone, 3uTools lets you do so. Download the ringtones on 3uTools, then import them to your iPhone. 

Operations:

Step 1: Start 3uTools, connect your iPhone to the PC. Click "Ringtones & Wallpapers", and click the download icon on the right side. Then you can import the ringtones to your iPhone after downloading.

Step 2: If you want to customize your own ringtones, you can make ringtones using 3uTools.  Click "Toolbox" →"Make Ringtone". Click "Created Ringtones"(Ringtones Completed), then select the ringtone to "Import to Device". Wait till the import is completed. 

Step 3: Click Settings →Sounds & Haptics →Ringtone, then set the ringtone you like.