Music is stored in resources/endmusic folder. You can add your own music, if you want.

WHAT MUSIC IS THERE NOW?
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Original music is created by Qwertygiy, the creator of More Music Discs mod: www.minecraftforum.net/topic/1032006-/


HOW TO ADD MUSIC
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1. You have chosen your music file. Make sure it's an OGG file, if not, you have to convert it.
2. Now decide, if you want the music to play when the dragon's calm, or angry. Rename the file(s), select one of available names (10 for calm, 10 for angry):
 CALM         ANGRY
  calm_0.ogg   angry_0.ogg
  calm_1.ogg   angry_1.ogg
  calm_2.ogg   angry_2.ogg
  ...          ...
  calm_9.ogg   angry_9.ogg
3. Put the file(s) into resources/endmusic folder.
4. Now, try it everything works, and enjoy your painful dying ;).

P.S. if you put there Justin Bieber's music, the dragon will rip off and sticks them into his butt.