Hey there! Good job on submitting the game :)
First of all, nice mashup of genres! I liked how the two gameplay mechanics mix up into a single game.
I like the concept, but I could not get how the left and right keys thing works. I tried holding left or right depending on the side of the sound, or just tapping once, or tapping for the whole duration, but I inconsistently got income on killing enemies. Thus it ended being impossible to scale my defense :( Maybe there is something I did not understand?
Anyway, as I said the idea is good and could be made into a really nice game!