Such a cool concept! From the very start the music and the looping mechanic gets you immediately hooked. I also enjoyed the mysterious atmosphere you created, and that breathing behind your back is soo chilling *_*
I had some hard time trying to loop the song though :D First I wanted to rewind the song just with my mouse, but it didn't work. Then I tried to start the new song in a new window as the first had ended, but also to no avail :) Only after that I understood how it was supposed to work haha
However if you close the window with the lullaby completely, the monster doesn't activate at all :D I didn't use that cheat as it made no sense to do, but this was a surprise x)
Regarding the puzzles I think it is really interesting how you rely on audio hints and use different means to proceed. The only thing that I would love to know more about is who we are and what are we trying to do! You probably planned on revealing it later is it is just a demo, but it would be really exciting to learn why these files are locked and what is our goal in unlocking them. Maybe we could do something with the creature after we learn the whole story?
A very solid entry!