MTV Geek! went original on the red carpet at the premiere, and asked the cast to sing the Thrones opening title theme. This resulted in some nervous laughs, paired with singing attempts of varying quality. This Tumblr entry gives you all of the cast in visual harmony (it may or may not be a good thing that there is not sound).
Hear Me Roar: Who do you think nailed it? Carice has a career as a singer going as well, and I think you can tell.