Hiya, I'm so glad you liked it! Hopefully Auggie's route will be done soon...! ^_T
The audio files should be easily accessible! If you want to save one, just remember to copy+paste instead of dragging it out, or else you'll get an error when the game can't find the file anymore :D
Open the Iris folder > game > audio > (music files)
The one you're thinking of might be called "Final Battle" or "Awakening" (to be renamed in the update).