The 90's: grunge - Nirvana, Pearl Jam, etc.
The 80's: new wave - Duran, Duran, tears for fears, etc is who i think of
The 70's: uh, that'd be disco - Bee Gees, Donna Summer, etc.
The 60's: psychadelic rock - Hendrix, the Stones, the Beatles, etc.
The 50's: Rock and roll - chuck berry, fats domino, little richard, etc. kicking it old school
You could make some arguments for the sounds of the 40's (Andrew Sisters) or the 30's (Brother can you spare a dime), but I don't think those are that strong so I'll leave them out.
Anyway, that means, that since the first time since the first half of the 20th century, a decade doesn't have a signature sound. In fact if i had to pin one musical signature to this decades music it would be American Idol - not really a sound, but a showcase.
Maybe this is because the decade isn't over and sounds of a decade only rise up looking backwards. Or this could be another result of the record industry is self destructing. Maybe its because music has stopped evolving - or the talent isn't out there anymore.
Or, most likely, its because I'm old and out of touch.
Dunno. But it seems like its hard to pin a sound on this past 8 years.