You are correct, Jonsi (the singer) is unmistakable. He sings partially in Icelandic but mostly in like, Icelandic-sounding gibberish.
The song is Untitled #9 if you were wondering. It's a B-side from their second album
They have a pretty cool "documentary" called Heima that they filmed while touring. They play some songs in interesting places with beautiful cinematography. VERY chill. I highly recommend watching it on a weekend when you're...uh...properly motivated to chill for an hour