Where did you get the music you're trying to move to SD? If it's downloaded from Google Play Music, you won't be able to move it to the SD card, because it's stored in a hidden directory with coded filenames. Music downloaded from Google Play Music is mostly intended for temporary use when you don't have a web connection, rather than moving around to other locations. If you want to do that, you're better off downloading your tracks form the Play Music website on your computer.
If you're talking about other mp3 files that you either copied to the phone from your computer or downloaded from other legitimate sites, then use a file manager app like
ES File Explorer.
