Yeah a bubbling sound is probably better. The biggest problem I had with the sound was that when I held the button instead of it replaying after it finished it would just restart the loop from the beginning and making this RNNNNN noise.
Yeah I read some of the comments after I commented and saw it stunned enemies which I had no idea. It's an interesting mechanic. Maybe an separate indicator for stunning might help.
Also unless you end up adding more levels or making the game really big, I don't know if you need a save feature. I didn't get to the end of this version but I imagine it isn't able to be beaten in more than an hour.
Most of all, I'm glad to hear you will keep working on it.