Oh my.... I did some custom Alpha Sapphire music... If you want, I can link you to that video. It should be the same steps as it is for Sun and Moon, due to it being both on the 3ds, and from what I can tell, both on the same engine. When I get my hands on Sun, I can make another guide and such.
Lemme know if you want to take a look at the video!
~Cheers!
Tepig.