Hello Julie: 

I'm afraid I don't have an answer for you about the rights to your uncles's recordings. Normally, those are negotiated with the record companies and music publishers (for composers). But with some of these small, independent labels, official documents are hard to find. 

In the case of the music in the Frontera Collection, we limit the audio samples for the public to 60 seconfds, to conform with applicable laws. 

If I may ask, are your uncles still alive? As Los Hermanitos Cordova, they sound like they recorded as children. The label is based in LA, so I wonder if they live locally. 

Thanks for your interest in our collection. 

Agustín Gurza, Editor, The Frontera Collection