This was a fun experience! I didn't have time to keep trying after I lost twice, stopped at wave 16, so I'm not sure if the game has an ending, but I definitely enjoyed it.
I believe the theme could be further implemented with timers on each enemy (counting down how long it will take for them to get to you), or bullets before reloading, or a backwards wave progression (starting at the last wave and ending on wave 1 or 0), or enemy count going down from start to finish of a wave (I realize the timer make it so there doesn't need to be any specific enemy amount, but still).
In any case, I felt the sounds and music was on point for the experience. With some more upgrades, more gameplay features, and an adjustment of the screen so they all spawn at roughly the same distance from the center (currently, enemies spawning from above or below spawn closer to the player than those who spawn from the sides or the corners), this could become a full-feature game in the future.
It reminded me a lot of old flash games that I used to enjoy, which is a nice thing.
Congratulations on your submission, and good luck!
P.S.: cut close to the deadline with this submission! Good thing you managed to get it sent in time.