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

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Sources and other great links:

Theodore Morris's video
   • Avatar The Last Airbender's Ingenious...  

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

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

An analysis of AtLA's music
https://avatar.fandom.com/wiki/User_b...

A reddit post listing off leitmotifs
  / atla_character_musical_themes_leitmotifs_i...  

Another thread just talking about how great AtLA's ost is
  / avatar_had_such_memorable_soundtrack  

Interview with the track team discussing some of their choices for the music
https://www.soundtrack.net/content/ar...

Another interview with the track team talking about the music
http://www.avatarspirit.net/interview...
http://www.avatarspirit.net/interview...
http://www.avatarspirit.net/interview...

Another blog post going over AtLA's music
http://gatorgala113animation.blogspot...

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.ph...
https://tvtropes.org/pmwiki/pmwiki.ph...