Can we use open source art assets in the game?

I'm not hosting this jam, so take this with a grain of salt, but: I believe reuse of existing assets is fair game. I've often used freesound.org and freemusicarchive.org for sounds and music.

and this usually applies to code, too; ex: if you were making something in JavaScript, of course you'd be allowed to install packages via NPM; if PHP, composer; if C#, NuGet...

Yep, we're more than happy for you to use (appropriately licensed and credited) pre-made assets as you wish.