I am not sure if I have to use music that is made for/during this Game Jam or if I could use just any royalty free music...

What are these two options supposed to be?

I'm planning on doing one of my own games for this but I love also making music for others. Please let me know if you'd...

What OS can we export to? I assume windows but I couldn't find it anywhere so I better ask before I do it wrong.

can only be done on the 4 engines described on the homepage?