LOS ANGELES—The television audience for Sunday’s Tony Awards for theater slumped to a new low despite a star-studded cast that promoted Broadway’s return after an 18-month shutdown due to the coronavirus pandemic. Broadcaster CBS said on Monday that just 2.6 million Americans tuned in for the two-hour special, which did double duty as an awards ceremony and a commercial for the return of live theater. In 2019, the last time the Tony Awards were held, some 5.5 million people watched the ceremony on television, which was the first time the show marking the best of theater fell below 6 million. Sunday’s ceremony took place almost two years after most of the nominated plays and musicals first played in New York City. Best-play winners “The Inheritance” and “The Soldier’s Play” both closed in 2020 and have no plans to return. “Moulin Rouge! The Musical,” which was named best musical and won …