So, the editor doesn't let you select the music theme for your custom race.

But you can edit this your self easily.

Open your custom race.xml file.

C:\Users\UserName\Documents\My Games\Elemental\Race

Under <RaceConfig InternalName="CustomFaction_Name">

Add <MusicTheme>****</MusicTheme>

Replace **** with one of these:

WanderersHome
BladeOfThePeople
StoneAndSoil
CrownOfTheWest
ForestRefuge
TraitorousEmpire
AncientShadow
EasternWardrums
SeeInDarkness

Music files are located in C:\Program Files (x86)\Stardock Games\Elemental\Layered Music if you need to listen to them before selecting one.

Enjoy!