Just for info: the name is "Palau de la Música Catalana" (Palace of the Catalan Music) -another video- and the resident choir is named "L'Orfeó Català" (Catalan Choral Society). The Palau was built around 1905 for L'Orfeó which had been founded in 1891. Here you can see one of the choirs of l'Orfeó on a performance, on Boxing Day last Christmas, singing a very patriotic song. There are several choirs within l'Orfeó, among which the main one, the adult's choir, and also the children's choir, the teen's choir and the chamber orchestra.
In general terms there's no dress code as such enforced, and except for very specific concerts (ie Christmas' or New Year's Eve for example), the typical dress code for the majority -as in most other venues in Barcelona, including the opera- is smart casual albeit these past years there is a trend in many to just dress casual (ie plain jeans). Still that depends a lot on one's 'background' if you catch my drift. Note in any case that 'casual' is not this, LOL! :))