Thank you for your feedback!
The game currently doesn't have any audio. (not a bug)
The number on top of the enemies shows the difficulty level of that enemy. So, as an example, if a diamond enemy has the number 7 on top of it it will fire 7 projectiles, or if a spade enemy has the number 5 on top of his head it will have a bigger attack than another spade enemy with the number 2. You get the idea :)