-Inti-illimani, legendary andean folk music group with social message.
-Ali Primera, Venezuelan Folk singer, and also a communist militant.
-Daniel Viglietti, Urugayan guitarrist and singer.
But according to your preferences, your looking for something like Inti-illimani, or Quilapayun.
I also have some FARC cumbias and vallenato if you prefer :P
"...Standing at a historic point, I will repay with my life the loyalty of the people. And I say to you that I am certain that the seed we have surrendered into the worthy conscience of thousands and thousands of Chileans, will not be able to be reaped at one stroke. They have the power, they can make us their vassals, but not stop the social processes, neither by crime nor by force. History is ours and is made by the people."
Salvador Allende, 9-XI-1973