While each floor at the Monarch Theatre comes alive with hip-hop and Latin music on Friday nights, the real fun happens in the upstairs lounge. Every week, a pair of 13-person Mexican banda ensembles, each equipped with brass section, multiple percussionists, and a singer, stage a battle of sorts in front of a packed dance floor. Banda La Alterada and Banda La Maciza each takes a turn performing a variety of ranchera and corrido songs. It's often a lively affair filled with fiery trumpet solos, audience members hopping onstage to sing along with their favorites, and DJ Fantastico spinning merengue and bachata before and after the performances. So who's the winner? Why, everyone in the house.