In an effort to bolster less-than-explosive ratings, "Miami Vice" executive producer Michael Mann has exhorted his writers to up the outrageousness in the show's scripts for next season. But in plotting for one of TV's most single men--Don Johnson's Sonny Crockett--to get married, well . . . Mann said Monday that Sonny will fall in love with a rock 'n' roll singer (so far uncast) and tie the knot. The marriage is an attempt to "emotionally involve our characters" more so than in previous seasons, Mann said. Filming for the coming season began in Miami last week.