I’ve watched quite a few of the matches in this tournament and the way it works both teams starters lineup on the field and each team’s national anthem is played, the tv shows close up shots of every player on the team.
The USA played Portugal today and every player on their team not had their hand over their heart but was singing enthusiastically, 3 of the 11 US players had their hand over their hearts and those 3 were the only players trying to sing
After watching a bunch of matches the US team is the only team acting this way
In my experience, it seems that athletes singing for the national anthem befor sporting events is more of a European thing — especially for those countries that have majestic national anthems. Check out any video clip of Wales when they are hosting an international rugby or soccer game if you want to see an awesome national anthem on display.