Congratulations for your fantastic historical site.
I'd like to add this music is called "El himno de Riego" (Riego's Anthem). It was the national Spanish II Republic (1931-1939) Anthem. It was the Anthem during the "Constitutional Triennium" (1820-1823), too, well officially since 1822.
It takes its name for General del Riego, who gave to a coup d'etat, proclaiming a liberal monarchy. The French absolutist invasion recovered the absolutist monarchy, Fernando VII executed Rafael del Riego in 1823.
If you are interested in music from the II Spanish Republic, you may visit:
If you want hear Nacionalist (Francoist or Fascist) music:
Unfortunally they are avaible only in Spanish.