I used to volunteer in a hospice. The "interview" was really more a chance for them to tell me more about what I'd be doing & for me to decide whether or not I would enjoy it.
They asked why I wanted to volunteer, what my future plans were, things like that. I also got a tour around the hospice.
In terms of dressing, smart casual will do.
I also went to a group information day, but that was separate from the interview.