Rounder Records, the pioneering American roots music label, is in the midst of its 45th anniversary celebration. Over the past months, the Cambridge-founded, Nashville-based Rounder has been making titles from its deep catalogue available as digital downloads. Last month, the label threw a Mardi Gras party with digital reissues of R&B, blues, jazz and Cajun music of the Crescent City. This month, the 51-time Grammy-winning Rounder - since 2010 under the umbrella of the Concord Music Group