Usually a moment of silence is held during removal of the casket.
Via WISTV:
A Savannah doctor was told not to sing the national anthem during a Delta flight that was carrying home the remains of a fallen Green Beret from Toombs County, Georgia.
When Pamela Gaudry learned that former Staff Sgt. Dustin Wright was on her flight, she and other passengers made an agreement to sing the national anthem as they waited for soldiers to unload his casket.
Gaudry said a flight attendant put a stop to the plans.
“I said, ‘it’s the national anthem’ and she said ‘it is against company policy to do that and so we are going to land and everbody is going to stay in their seats and be quiet,'” Gaudry said. […]
“Delta has contacted me – no money or free tickets (I promise, and I would not have taken them) – and this is not their policy,” she wrote on Facebook. “Evidently, they had a flight attendant that made some bad decisions in trying to make this situation go away.”