Cool game, I love it! Somehow managed to win on the first attempt.
I like the enemy behaviors and the boss mechanic. Regarding the boss i have a suggestion: delay when the attacks affect him until he has fully risen, that makes sort of a build up while presenting the boss, making more it more.. sort of a "you can't touch me!" epic raise. :)
About the audio, I set it usually by default to 50% volume, in case that helps, my system's audio is at 30 but people usually have it maxed at 100, so i can see how it could blow someones ears if set to maximum by default. At the very least, I'd suggest adding a volume slider, even if it doesn't get saved in between game sessions, unless you intend to complete the game and publish it, which at that point i'd consider it a must-have.
Cheers, and i hope this helps :D