Thank you for the kind words, we're happy to hear that !
Yes, Call of Duty Zombies was obviously a big inspiration, but we didn't want to stop there, so we also mixed in ideas from other games that we really like.
As for the difficulty curve, we're definitely working on it. We already have a lot of ideas for new mechanics and more gameplay depth, and we're planning to add new enemy types, more events, and more.
For the zombie sounds and the SFX in general, we did run out of time, so those will be reworked too haha.
And you're right about the second level: we changed its label from "Locked" to "Coming soon", since it was confusing, we just put it there so the level selection would be visual !
And just a small technical note: we did start everything from scratch when the jam began. I personally designed the 3D models in MagicaVoxel (props, structures and everything), and my friend 0xi0de handled the character and zombies models in MagicaVoxel, plus the pixel art in Aseprite. For the VFX we used it's all in the credits ! :)