Why would fonts or sounds be in variable memory? Aren't those constants? And how do images fit into this?

Similarly, what does the current display count as? Is reading the screen to see what pixels are currently showing where break the restriction on variable memory?

Yeah, I'm confused about this too. I can't think of a good way to follow the rule.

Use those 64 bits only as numerical storage. All other things can be exempted from the rule. Yes, I might have not been clear.

64 bits, or 16 bytes, as you want. Use a variable with that size and use bitwise operations alongside other variables to manipulate it in your favour.