In some situations it might be better to put the board in 3 parts. power and sd card for one board, led and propagation on one board, sound amplifier and speaker connections on another board.
I suggest considering placing the electronics in the front of the blaster, instead of the back.
batteries can also fit inside the front, or even in the handle of the blaster itself.
you could also consider making 2 areas for batteries. I'd also like to see a charging system for batteries.