This show has some of the best music ever. I've always been really sad that ATLA never got a proper soundtrack release. Jeremy Zuckerman said he would love to release ATLA's music in his Reddit AMA. However, I guess they recorded all the music on old equipment (not all digital), and he has to transfer it all over, clean it up, and possibly re-record some things. And then even after that, he would need to get support from Nick to release it.
Sounds like he's interested in doing it, not sure if he'll get the support to release it. At the very least, I would hope they release it digitally on iTunes.