Hi, idk how to answer this because I have the same question!
I love volleyball and I want to do my project on it too. I think maybe we are going to play loud music vs. No music and see which one helps you make the ball over the net.
Here is link from where you can find the ideas on sports for a science fair project. Have a look:
I did this science fair project where I used different serving techniques to find out which was the best for the most accurate serve. It turned out pretty well.
I think you should see which volleyball bounces the highest, thus, seeing which volleyball is best to play with.
Here is a link which contains sports medicine science fair projects and experiments. There are topics, ideas and probability strand of math and a specific sport like volleyball. Hope it helps.