Some schools require medical school applicants to have volunteering experience, so research those requirements for the med schools you intend to apply to. For example, Harvard Medical School does not specifically state volunteer experiences as a requirement to apply, but if you want to be considered you should volunteer.
Meanwhile, Weill Cornell requires extracurricular activities as a prerequisite. You can technically apply without volunteer experience, but it significantly lowers your chances of getting accepted. Check with the med schools to which you wish to apply and see whether or not volunteering is required as one of the extracurricular activities.