NEW YORK — About 4 1/2 hours after the first notes of Wagner’s “Tristan und Isolde,” a startling sound emerges from the wings, one many in the audience likely have never heard before. A wooden holztrompete joins the Met’s new ‘Tristan und Isolde’ production A nearly 4-foot wooden horn known as a holztrompete, specially constructed to the composer’s somewhat ambiguous specifications, signals the arrival of the ship carrying Isolde and King Marke to Brittany, inspiring a mortally wounded Tristan to hang on to life for a few more moments. “Joyous,” said Billy R. Hunter Jr., the Metropolitan Opera’s principal trumpet, who plays the wooden horn from stage left. Yuval Sharon’s compelling new production starring Lise Davidsen that opened Monday to mostly rave reviews features a specially constructed horn nearly Hunter’s height — it measures a minimum 46.5 inches and lengthens slightly if the tuning slide is turned. “You listen to the sound of the holztrompete and the imitation, it’s a clear difference,” said bass-baritone Ryan Speedo Green, who sings Marke alongside Davidsen’s Isolde and Michael …