I'm a singer. I've been considering getting my balls chopped off for a good while now. Might this have any effect on my voice, lungs, diaphragm, or sinuses? I'd hate to have to learn to sing all over again.

You should be positively sure that you want to be castrated, otherwise you may be sorry for the rest of your life. It is a life changing experience, think about it carefully before you go through with it. If you are sure, here are my thoughts:

If you have gone through puberty, your voice will not change after you are castrated. Some eunuchs state that there is a very slight change with some voice cracking, but overall, the voice will not change. Even in transgendered persons, to get the voice to change requires surgery to shorten/lengthen the vocal cords. It also may depend if you are going to use testosterone after you are castrated. If you have a high tenor voice, your voice may get a little lower if you use high levels of testosterone. But I've never heard of a bass going up to a tenor after castration. As far as your diaphragm, sinuses, etc., you might lose some lung capacity unless you use HRT. Testosterone provides strength, and that includes lung strength. Hash