You're right about that sound; it was the first one I made, using raw WebAudio, before I decided to use sfxr, so it was a bit out of place and bad. I've done an SFX pass and all the sounds are sfxr now, and should all hopefully be passably nice to listen to.
Yes, the sounds make it much butter! Just a little more feedback and a nice pling sound for pickups makes a big difference. This is a really fun little puzzle game!
I've tried your game out and I like it so far!
Suggestion: The current Death/Fail sound is very harsh. Maybe you could try something a little softer, not that alarming?
You will certainly be hard to beat on file size, great job!