From Sideways.

Long ago, all the leitmotifs lived together in harmony
Then everything changed when the fire nation attacked

Twitter: https://twitter.com/Sideways440
Twitch: https://www.twitch.tv/sideways440
Patreon: https://www.patreon.com/Sideways440

Sources and other great links:

Theodore Morris’s video
https://www.youtube.com/watch?v=KyuCigqn9yc

A list of leitmotifs in AtLA
https://avatar.fandom.com/wiki/Soundtrack_of_Avatar:_The_Last_Airbender

Diegetic music of AtLA
https://avatar.fandom.com/wiki/Music_in_the_World_of_Avatar

An analysis of AtLA’s music
https://avatar.fandom.com/wiki/User_blog:Uiop60/Analysis_of_Music_in_Avatar:_The_Last_Airbender

A reddit post listing off leitmotifs
https://www.reddit.com/r/TheLastAirbender/comments/6jb3a2/atla_character_musical_themes_leitmotifs_in_atla/

Another thread just talking about how great AtLA’s ost is
https://www.reddit.com/r/TheLastAirbender/comments/8l0b3y/avatar_had_such_memorable_soundtrack/

Interview with the track team discussing some of their choices for the music
https://www.soundtrack.net/content/article/?id=252

Another interview with the track team talking about the music
http://www.avatarspirit.net/interviews.php?id=2
http://www.avatarspirit.net/interviews.php?id=3
http://www.avatarspirit.net/interviews.php?id=4

Another blog post going over AtLA’s music
http://gatorgala113animation.blogspot.com/2016/04/the-music-of-avatar-last-airbender.html

And honestly, tvtropes had a few things to look out for, worth looking at if you want an easy place to start
https://tvtropes.org/pmwiki/pmwiki.php/Leitmotif/WesternAnimation
https://tvtropes.org/pmwiki/pmwiki.php/AwesomeMusic/AvatarTheLastAirbender