Everyone seemed to be enjoying, if not exactly understanding “RoosevElvis’’ at the American Repertory Theater the other night when, about 40 minutes into the show, the voice of the stage manager came over the speakers. “We’re sorry but we have to suspend the show. Please exit the theater.’’ But because the show at Oberon has a kind of hallucinatory quality, the audience wasn’t sure if it really was supposed to leave. It was. “We’re stopping the show,’’ said the voice again. “Please exit the theater.’’ At the door, patrons were told they would be receiving an e-mail about a refund. A spokeswoman for the theater told us Monday that Kristen Sieh, the actress who plays Theodore Roosevelt opposite Libby King as Elvis Presley, had become sick and couldn’t continue. She’s feeling better now and the show goes on — until May 29.