Another place that is usually free, and sometimes has live music, is the studio of Carlos Reggazoni, the crazy found object sculptor.
His studio is on Libertador, just to the west of the Retiro train station, up a little alley. It is right next to the train museum. At least in May and June, they were doing live music on weekends, very casual, with drinks available, in a very bohemian artist studio/gallery atmosphere. A mere few blocks from the olgiarchs in Recoleta, but a world apart- almost like a beatnik gathering in the 1950's.
http://www.regazzoniarts.com/