"Raise the Roof," Robert Plant and Alison Krauss (Rounder Records) Pairing Alison Krauss' angelic soprano with Robert Plant's roguish tenor once sounded like a bizarre idea - until they started to sing together. The improbable partnership produced the hit album "Raising Sand" in 2007, and their chemistry clicks again on the long-awaited follow-up, "Raise the Roof.".