Raising Sand, the collaborative effort of Led Zeppelin frontman Robert Plant and bluegrass diva Allison Krauss, plays like the bedroom negotiations of two aging lovers. Where once Plant’s ballads dripped with post-coital tenderness, here, they soothe the “not tonight, my head hurts” apathy of age. Most of the tracks quietly, slowly, with hints of sorrow or love, mark the passage of uneventful nights.