They appear to be defined in CoreLayeredMusic.xml. They are linked from the raceconfig

<MusicTheme>BladeOfThePeople</MusicTheme>

and in CoreLayeredMusic.xml you have stuff like this

  <LayeredMusicGroup InternalName="Theme_BladeOfThePeople_Full">
    <MusicLayer InternalName="BG">
      <FileName>Theme_BladeOfThePeople_BG</FileName>
      <Volume>100</Volume>
    </MusicLayer>
    <MusicLayer InternalName="MG">
      <FileName>Theme_BladeOfThePeople_MG</FileName>
      <Volume>100</Volume>
    </MusicLayer>
    <MusicLayer InternalName="FG">
      <FileName>Theme_BladeOfThePeople_FG</FileName>
      <Volume>100</Volume>
    </MusicLayer>
  </LayeredMusicGroup>