[Eromaveabeara]

When the national anthem of your country is played; say at a sporting event.

Is it haram for a muslim to stand up and show respect?

Good question.
I suppose it will depend upon the wordings of the anthem.
God save the Queen is a supplication and would seem alright to yours truly.
Indian national anthem is a set of few stanzas from a poem written in praise of the then king of Great Britain George V.
It includes phrases that assert the Kind George V is the decider on the destiny of India which is clear kufr.
Allah (SWT) is the decider of everyone's destiny.
It is possible to wiggle out of the situation by asserting that we are no more referring to King George V as the decider of India's destiny. We are now referring to God as the decider of the destiny of India.

I say that the question is good because all the national anthems have diverse backgrounds and all the backgrounds are ideological. Ideologies usually clash with faith. Clearly mater has to be decided on individual basis. And you do need Muftis to do that. So I keep my peace.