UNIVERSAL CITY, Calif. (April 29, 2011) -– NBC revealed that it will add more star-power to its new vocal competition series “The Voice” with singers Reba, Monica, Sia (Furler) and music producer Adam Blackstone joining the series as new advisers who will lend their experience and talents to musician coaches Christina Aguilera, Cee Lo Green, Adam Levine and Blake Shelton for the next “Battles” competition phase. “The Voice” next airs Tuesday, May 3 (9-11 p.m.