I am not much fun on this, as I tend to follow jazz. Keep an eye out for Luar na Lubre, a very succesful Galician Celtic music band, as they frequently do gigs in Galicia and northern Spain. Santiago is a town of students and young people and I have seen semi-traditional bands and dances set up in the city, such as in the Praza de Feixoo. At a nearby bar, accompanied by a German handball captain and an Australian wine merchant (hola Adrienne!), I wandered into a performance by TéCanela, who energized three really really exhausted pilgrims that night. At our reunion three years later Ariadna Rubio's voice rang out from a cabin in the Blue Mountains near Blackheath a month ago, cheering up local marsupials.
If you hit a fiesta, there will be dancing in the streets, but you cannot always count on the music being folklorico. In Lugo I was witness to about 400 senior citizens (I lost count after 300) swingdancing in the main square.
In the way of Spain, ask the hospitalera, ask the bartender, ask a young waiter or waitress, maybe even the Guardia Civil!