Volunteering isn't always as simple as you may think. If you go over to Korea to teach as a volunteer, then your school will still be responsible for you even if they don't pay your wages.
Eg: If you can't pay your rent, can't feed yourself, then the school will be the first ones you talk to. No-one wants to be put into this situation.
That's why a lot of volunteer programs do in fact require a fee. See
http://www.traveltree.co.uk/voluntee...h-in-korea.htm
This fee then helps towards looking after you when in a foreign country with no income.
Am sure there are still a few schools that are willing to employ you without a visa on a part-time basis... But it wont be an easy ride for you. Why don't you just go to Korea for 1 month holiday? See how the country is first?