1. Can I use any free assets not made by me nor during the jam? (for example, royalty free music)

2. Can I start working on the art for my game before the jam starts?

"Yes! If you have the rights to use it, anything that wasn't made specifically for this particular jam is allowed." - The game jam rules

It would be a good idea to fully read the rules before posting. :D

I've read the rules, but I wasn't sure if my 2nd question wasn't violating this rule: "No. You may only submit projects made within the time frame. We do check.", as it's essentially working on a part of the game before the jam starts.

I think you can make consepts, but I dont think you can use those during the jam. Maybe you can make stuff that you might make, but you can't use the same 

thing during the Jam.