Here in the dwindling nights of the American Composers Orchestra's aptly titled showcase SONiC: Sounds of a New Century, composers aged 40 and under are filing out of venues in both Manhattan and Brooklyn, all of which have played safe harbor to their thoroughly modern machinations. Saturday night's performance at National Sawdust, care of the unflappable vocal ensemble Roomful of Teeth, duly certified festival, venue and ensemble together as one hell of a discovery.