If my mind serves me correctly, I am sure it says in the branch correspondence guidelines or the elders secret hand book -shepherd the flock of God- that it is acceptable to stand for the national anthem out of respect, but not to take part by singing.
I remember being peeved when I read this as I had a couple of embarrassing situations when the national anthem was played on some nights out. All the witnesses remained seated and we all looked like a bunch of disrespectful assholes.