I wholeheartedly agree with the music thing, I once played it "vanilla" and once with https://www.youtube.com/watch?v=a6BbvCC0VI0 playing in the background and the difference was immense.
The only part of the demo I didn't like was the fight against the two flying eyeballs, these enemies felt a little overpowered for random enemies, being able to kill you near-instantly if they both attack at the same time. Though I guess they were this demo's bosses, so it makes sense in context.
Can't say I have much else to note, everything ran smoothly during gameplay. Once during loading Window's "This application has stopped responding" popped up for a second then disappeared when loading finished, though that may just be on my laptop. My favorite weapon is the vinyl launcher, I think bouncing discs of the walls and hitting enemies with that is pretty fun.
I never found out where the yellow key goes, maybe I used it without noticing it. I like it that way though, I prefer automatic item use over manually selecting key selection in action-type games. Looking forward to the next demo!