Thailand is one of the few monarchies (like Denmark and Sweden), which has a separate National Anthem for the royal family as opposed to the national anthem for its citizens.
The Thai Royal Anthem, is called the Sansoen Phra Barami and is performed during state occasions and before a film in the cinema begins. When the Royal Anthem plays in the cinema, it is a criminal offence not to stand and take in the short movie clip about the King. Sansoen Phra Barami was the national anthem of Siam until 1932, when it was replaced by Phleng Chat. The royal anthem is performed during state occasions, as well as when a high-ranking member of the royal family is present for a function.
Source: https://en.wikipedia.org/wiki/Sansoen_Phra_Barami, http://nomadicboys.com/facts-about-thailand/