I actually went and tried to make a pretty complete list of songs that I have headcanonized as theme songs for particular Austraeoh characters because screw reasons. Lots of Miracleofsound and Florence + The Machine of course. Sometimes the same song fits several characters. I'll just dump all those that I remember right now here and then go from here. Have fun.

Rainbow Dash
Spark, the Austraeoh, Seed of Rebirth
The Dawnbound, Daugher of the East (Thanks, Arc!)
Mother of Awesome, Duchess of Dashing, Lightning Incarnate
Essence of Harmony, Light of the West, Avatar of Loyalty
The Lonesome Spark
Beloved No More

Roarke
The Metal Mare
The Rarity, most Righteous

Nightshade
Victim
And Villain

Shell
Hollow One, Shadow of the Spark, King Midas of Ledomare

Zzap Naator
Duke of Darkstine, Slayer of Children, Guardian of Industry

Axan
Divine of Fire, Mother of Dragons, Bane of Silvadel

Bellesmith
The Good Doctor, Wind's Beloved, Eljunbyro
The Bloodied Lamb, the Dying Wind (Thanks Arc!)

Pilate
Eyes of the Jury, Wind's Beloved, Eljunbyro

Josho
Swimming Once More

Kera Tin Mehjj
Storm Child, the Windswept
The Clover Uprooted

Ebon Mane
Son of Queen Ledo, Prince of the Seas, Conqueror of Hearts
....also, Suspected Changeling

Floydien
The Volatile, the Glimmering Keeper
The Twice Forsaken

Eagle Eye
The Blood-Stained Butterfly
Sexiest Colt Alive
Blossom in the Wind

Imre
The Fatherless

Tweak
Crystal Slinger, Bane of Searo
Sapphire Farmer

Nevlamas
Nagu'n, World Eater, Dark Divine, Mother of Xon

The Noble Jury
Shelter
Home
Center of the Action

The Proto-Pegasi
Angels Still if Birds or Devils

I have some ideas that fit the Austraeoh series in general, but mostly in spanish.

Roarke
The Metal Mare

I swear I personally suggested this one in connection with Roarke a long while back.