[quote=@TaintedMushroom]
I'd love to hear more about this. Have you read any of the Chronicles of the Avatar books? I haven't myself but I was doing a bit of research on the concept of soundbending after you mentioned it and it's something that apparently avatar Yangchen might have developed well before Aang's era even. My only concern basically is how the whole apostate thing comes about, admittedly I don't know enough on air nomad lore myself. 
[/quote]

I have not read the Chronicles of the Avatar, so I wasn't aware that soundbending already existed.

His apostasy happens due to several factors. One of his parents is a toymaker, inculcating him with a desire for 'efficiency of craft'. He didn't realize he had airbending capability until he was a little older than most, so he joined the air temple as a young adult instead of as a child, so he did not absorb their teachings from a young age. he was seen as something of a slacker, since he ignores 'discipline' and 'training'; if he already 'knew' how to do something, he didn't see the point of repeating the same motion a hundred times. He discovers soundbending and starts experimenting with it, which initially amazes his teachers. But when it came time for him to find an air bison partner, he emits a sound that is alien to the air bisons, scaring them. Since air bisons are sacred, this worries the air temples and they ask him to temporarily cease his activities. At this point, he has become obsessed with his new technique and continues developing it in secret. To test soundbending's efficacy as a martial art, he purchases animal carcasses and skeletons and practices on them, while also making 'soundbending tools', ie. musical instruments (using techniques he learned from his toymaker parent) which require animal parts. This violates the airbender's tenet of valuing the sanctity of life; even though he did not kill these creatures himself, he is financially contributing to their slaughter and using their parts in unceremonious and somewhat grotesque ways. This gets found out, which only cements the idea that soundbending is an evil practice in the eyes of his teachers. In an act of defiance, he shaves his head, declaring himself a master of his own school, has a confrontation at the air temple with his former masters, and steals one of the temple bells before apostating, becoming a deviant and hermit.