Your volunteer time doesn’t have to be related to your major in any way, shape, or form. They—and future employers—just want to know that you are interested and involved with the community. If there isn’t anywhere to volunteer that you are interested in their cause, then you should fall back on skills learned in school and you can count it as an unpaid job.
To give an example: as a business major, I volunteer sorting books for a non-profit bookstore. I love the the charity I work with and it has nothing to do with my actual major. But it still shows community involvement that may have been important in getting into business school.