The music is enchanting.     This cast of 12 sounds like double that. The dozen actors perform the slapstick, the dancing and the singing effortlessly and on a very grand scale. Audiences loved all of them; the filming of the horseback riding, the bathing beauties, the Keystone cops, the pie-throwing episode and more. The superb ensemble included all standout actors.  Their actions sometimes were literally louder than words, and the ensemble singing was extraordinary.     Musical ensemble numbers abound and it’s hard to pick a favorite. “Tap Your Troubles Away” (performed by Brook, and the two Paquins) was a true highlight which brought spontaneous hoots and hollers from the audience with the very first tap.     Cape Rep’s “Mack & Mabel” is the complete package: brilliant performances, sparkling song and dance numbers, and an intriguing storyline. Don’t miss it.         THE CAPE COD CHRONICLE by Amy Tagliaferri