The thrill isn’t exactly gone with Sigur Rós, but a dozen years after their breakthrough U.S. LP,Ágætis Byrjun, the breathtaking elegance of their music has sorta worn off. The celestial Icelandic band – best known for sculpting dreamy aural landscapes and occasionally making up its own language – is still one of the world’s most exciting groups, if a catalog of songs that can double as musical Ambien can be called exciting. Valtari, their sixth album, returns to grand atmospheric set pieces after detouring on 2008’s disappointing Með Suð í Eyrum Við Spilum Endalaust. Songs like the elegiac “Ég Anda” and “Ekki Múkk” – which run six and seven minutes, as do most songs here — slowly build on whispering electronics and Jónsi’s ethereal vocals. Settle in for the big chill.