Imagine a packed stadium, the roar of the crowd swelling as the national anthem plays. But instead of voices raised in song, all you can hear are hums.

The “Marcha Real”, translated as “Royal March”, has been the anthem for Spain since the 18th century but, unlike other European countries, it lacks lyrics for its people to sing to. Composed in 1761 by Manuel de Espinosa de los Monteros as a military march, it won the Royal seal of approval from King Charles III in the 1770s. Since then, it…