It might just depend on the engine you're using or how the audio is being played. Typically most audio is 44.1k but sometimes it can be 48k. Is the audio being streamed instead of loaded in? Honestly I don't really know *that* much about audio but ive had issues before with streaming large audio files before. Overall, IDK though. lol. Might just have to try some different settings to see what fixes it. It could be an HTML5 issue with whatever engine you are running for certain sample rates. haha.
As far as difficulty I found it gets quite difficulty a bit to fast maybe. I was only able to get like 3 powerups before I became quite overwhelmed, so maybe just slightly dulling the spawn rate in the beginning of the game a bit? That could also just be because I suck too, so take my opinion with a grain of salt. lol. Depends on the type of audience you want playing your game I suppose. I'm not really good at games so I tend to prefer more relaxed difficulty curves, but that is just me. haha. :)
Great entry though! Well done! :D