I recently learned about two more venues offering free concerts in the city.
Colegio de Escribanos de la Ciudad de Buenos Aires
Thursday concerts at 19,30 hs.
Today's concert is the Cubil Orquesta (big band jazz)
Av. Callao 1542
Jockey Club has concerts on various days and times. You can get a schedule at the entrance on Libertad. The Alvear entrance is for members only.
Wednesday, May 22 at 19 hs
Violinist and pianist from Estonia in recital
Neither venue publishes a schedule of concerts on the internet.
It's the same case for the Fundacion Beethoven, Santa Fe 1452
A friend sent me this information.