Peeking into the rock

Because I'm a big old geek or something, I want to learn to sing the Levan Polkka to sing to the babies.

But I don't know what doubled vowels and doubled consonants do in Finnish, or what they use the umlauts for. Worse -- because I can't begin to imagine what they sound like -- are the doubled umlauted vowels. Anybody know how to tell me, or should I ask in linguaphiles?

( rambles in baby entertainment below the cut )