Doing it too cheap, part sponsored gig.
That means I have some say in what happens. I have given them 3 options.
Perform acapella outside under cover while the guests are having pre entry drinks, recorded walk in music, then perform the national anthem inside on stage once everybody is seated and quiet.
Sing really loud inside while guests try and find their seat, yell across tables at each other and let me have volume wars with the punters who just get louder as the music does, to a point I can go no further and the crowd drowns them out.
Stay at home, we will get a soloist to sing the anthem, and play recorded walk in music.
Hopefully they go for option one. These are primary (elementary) school choirs, they are completely unable to project in any fashion whatsoever, most of them just mouth the words except for the chorus.