Forget cowbell--songs need more vocals, or at this weekend's SOJAM A CAPPELLA FESTIVAL, only vocals. The festival will draw a cappella groups from colleges across the Southeast, including the Duke Pitchforks and the Elon Sweet Signatures, and national acts Duwende, Cadence and Almost Recess. This is not a barbershop quartet convention. In recent years, a cappella music has carved out a nice niche within contemporary music, with vocal percussion and driving, wailing rock made with no instrument but the human body. Performances are Friday ($22, $14 for students) and Saturday ($32, $22 for students) at 7:30 p.m. at the Progress Energy Center in downtown Raleigh. For more info, see www.sojam.net.