Stephen Sondheim’s 1981 Broadway flop Merrily We Roll Along is a show I love. Yet the mostly inept new production (at the Laura Pels Theatre through April 7) made my eardrums throb and my eyes wince. The director, Noah Brody, uses a single set that is meant to remind us we are in the realm of theater. Any director whose principal idea of theater is to remind us that we are dealing with theater should probably return to the theater manual and try to get past chapter one this time. The aggressive ugliness of Derek McLane’s set, suggesting an immense backstage area in which warehouse-sized shelves groan with props and costumes, is rivaled in unpleasantness by a nondescript cast whose members should earnestly explore alternative career paths.