Not too sure about this song in particular, but in some of their songs Sigur Rós aren't speaking any language. They made up their own 'language' just to suit their songs, using voice as an extra instrument. So it doesn't mean anything, really!
from:
http://www.sigur-ros.co.uk/band/faq.php
"what language does jónsi sing in?
on von, ágćtis byrjun and takk, jónsi sang most songs in icelandic but a few of the songs were sung in 'hopelandic'. all of the vocals ( ) are however in hopelandic. hopelandic (vonlenska in icelandic) is the 'invented language' in which jónsi sings before lyrics are written to the vocals. it's of course not an actual language by definition (no vocabulary, grammar, etc.), it's rather a form of gibberish vocals that fits to the music and acts as another instrument. jónsi likens it with what singers sometimes do when they've decided on the melody but haven't written the lyrics yet. many languages were considered to be used on ( ), including english, but they decided on hopelandic. hopelandic (vonlenska) got its name from first song which jónsi sang it on, hope (von). tracks 7-9 on takk are in hopelandic."