After sampling a ton of the music channels on XM, I suspect XM13 - Willie's Place, probably best represents the music that Jack and Ennis would have spent listening to over their history together. It's considered a honky-tonk of sorts, with a lot of western swing and old school country music, and it usually has a live announcer as well (recognized because during a technical fault last Friday, the announcer came on to apologize for it and was manually playing CDs until they got the computerized music system working again).
Of course, XM and Sirius have a range of country music outlets, but most are playing songs recorded much more recently.
Brokeback Mountain made the life changing event in me of actually tolerating country and western music, something I dismissed before the movie. These kinds of little things are what still touch me, some eight months post-Brokeback Mountain. And that is a sign of the seed that was planted by the film that changed me, not just because of the story up on the screen, but because it gave me a renewed sense of respect for the culture of the west, and a new interest in learning more about it.
It will be things like this that will likely carry with me for the rest of my life - a sense of permanent change brought about by an independently produced and released film.