2 questions

1)When I have been riding my trials, it makes an awful grating sound whenever I push down the right crank. One of the spokes is missing and the nipple disappeared inside the rim, it is a nice rim so is there a solution?

2)I am guessing it is something with the hub and I wanted to get a better something, the qu ax is tempting (but I do have a 36 spoke rim) but is there a way to get a 36 spoke one? the kris holm splined and and a suzue hub, does anybody have any other ideas? I am not focusing on real trials as much as street riding, I need something strong to avoid replacing it again while I slowly work my way up.