Thank you for trying out the game. The music is from opengameart - the link is in the game's page somewhere below. There are plans to put a couple more hours into polish to flesh it out.

Thanks for playing! Yes the spike in difficulty is ridiculous going forward. I am planning to do some balancing and polish work later.

The music I used is linked in the game's info if you really like it.

That is true, some tinkering was needed with the projectile speeds or rather the rate of firing + number of guns to be spawned. Regardless, thanks for playing!