ohhh creating normals can be pretty tricky--if youre feeling adventurous one possible solution may be to find a grayscale map > normal map program (i think xnormal was an old one) and see if the results of that work. at worst, drawing them by hand is possible, but i probably wouldn't recommend it unless your tiles are simple + you have time to iterate and make sure the colors and vectors are correct.
personally i start to work with audio depending on the type of project--for text games i might not really think of it until the end, for VNs i try to have in mind the kinds of tracks or sfx we might like toward the middle of development, but for 3D games i find sometimes it's pretty important to immersing a player in a world and testing pretty early on. honestly this isn't best practice haha but i've sometimes considered audio a reach goal for a jam game, but for other projects i think it can help to look into it sooner than later.