People did this.
https://www.euronews.com/2019/06/17/watch-hong-kong-protesters-sing-les-miserables-do-you-hear-the-people-sing
There’s also a video of the singing of the US national anthem while holding our US flag.
Our anthem being sung while holding our flag: https://www.redstate.com/brandon_morse/2019/08/12/hong-kong-protesters-wave-american-flags-sing-national-anthem-demonstration/