It's very unlikely that you'd get volunteering experience just on a lab, or even getting to step on a hospital lab in the first place as a volunteer. Hospital volunteering tends to be more of a healthcare assistant nature rathen than anything else. Still, no harm in trying! Some hospitals list volunteer opportunities on their websites, others will have a special email address you can write to. You might be able to get the email address of a doctor from your local hospital, if you do you might want to consider having some correspondence with him/her (if they're inclined to do so) and ask him/her for volunteering or work experience opportunities!