The Belasco, 1050 South Hill St, Los Angeles. May 15th. Cumbiatron — the Cumbia Rave — at the Belasco, which is the Downtown LA venue that handles this kind of night at the right scale: the space, the sound, the crowd that came specifically for this. Cumbia is one of the most durable rhythms in Latin music — the bass pattern, the accordion, the percussion that locks into something your body knows before your brain catches up. The rave format takes that rhythm and puts it in a room designed for dancing, at volume, with a crowd that has been waiting for exactly this configuration all week. The Belasco floor …