They are singing "kumikyoku nico nico douga" to commemorate the first anniversary of nico nico douga.
from nico nico douga
www.nicovideo.jp/watch/sm3696817
※ I'm not original uploader.
And you can see the list of songs used in this medley. Come to see my profile page.
動画お借りしました。
26 ноя 2008