To find voluntary activities related to arts in the capital area you could contact Maailmanvaihto ry :
http://maailmanvaihto.fi/en/volunteerin ... placements
( the page is in English )
Volunteers working at a social project have the opportunity to work at a foster home or at an organization that works with people with disabilities. These projects aim to provide their inhabitants with family-like surroundings, close contacts, as well as advance their development and sense of security. Volunteers normally learn about and assist in various types of therapy, art classes, handicrafts, workshops, agriculture, garden work, animal care, and more.
You could also send your query to KVT ( the Finnish branch of Service Civil International ) in Helsinki :
http://www.kvtfinland.org/index.php/kvt-in-english.html
( the info on that page is in English )