Although it is not legally specified, performers of the National Anthem are generally expected to behave in a fashion that reflects the solemnity and honor of the occasion. Performers should sing the song to the best of their ability, without significant deviation from the original notation.
The general protocol for conduct during performance of the National Anthem is described in U.S. Legal Code, Title 36, Subtitle I, Part A, Chapter 3, § 301. It specifies that members and veterans of the Armed Forces should hold the military salute through the entirety of the song, and all other individuals should remove head coverings and hold their right hand over their heart. All spectators are expected to stand and face the displayed American flag throughout the performance of the National Anthem.
The National Anthem is traditionally performed at the beginning of public sports events, orchestral concerts, important political events, and other large public gatherings. If other nations are represented with the performance of their anthems, the foreign anthems are to be performed first out of respect.