Two ways of getting it:
Win the Sound Tournament which is hosted by me atleast two times a year.
Become the student of following people:
Xylon, Mugiwara, RokuNR, Kaito.
Nero was the one who was teaching Sound, me and Kaito were the last people he passed it down to. So me and Kaito were the one who had power over sound, but Kaito didn't want anything to do with it(? correct me if I wrong, but thats what I got from Kaito), so I decided to distribute it through Sound Tournament.
After each sound tournament, the winner + the student chosen by the people listed up there can learn it.
Gobi won the tournament, and Xylon wanted to teach it to Demosthenes, I agreed to the deal that Xylon would be able to pick a person student after each Sound Tournament and teach him Sound, if I was able to do the same thing, but that he would teach my student as well. Wesobi was my student. Recently RokuNR wanted to teach it to one of his students, he was allowed. Kaito had the right to teach his own personal student from day Nero finish teaching us sound(he sent everyone that wanted to learn sound to me, for the sound tournament, but he can choose his own student). Don't remember Ali asking me for permission to teach people sound. Right now, I'm not even sure I still have those privileges as I'm no longer a mod. But I believe the rules that were made during the time I was mod still stand.