Can I go back out of volunteering?
Did something urgent come up? It's up to you.
No, I just want more freetime.
If you agreed to do something, you should see it through to the end. You already promised to volunteer and they are expecting you to help. If you suddenly cancel, they will struggle to accomplish all of their goals and will be annoyed with you.
It's nothing, I just don't want to do it anymore.
I think maybe you should just find it in your heart to do this volunteer work one last time. Obviously, it sounds like you don't enjoy it anymore, so you might as well give it one last hurrah. You might need them for future job references, so I suggest you keep your promise and finish the job.