To most ears, the police siren is simply a cacophonous necessity. But Mexico-born musician Lázaro Valiente — better described as a purveyor of funky, found sounds — appropriates that wail and other cop-car noises for an amusing, chaotic, and unusual experiment in composition. For example, in Police Car Quartet — the first of nine public happenings with similarly pithy titles like muchos Mexican barrel organs and visual concerts at red lights — Valiente turns variously pitched slides and blips from four cars into a symphonic siren-ade. Of course, the question remains: how did he sweet-talk the authorities? Tonight, the artist speaks, performs, and doles out some artworks.