JAMA is the third journal from the group of "big five" in medical publishing to feature audio summary of content. "Big five" consists of NEJM, JAMA, Lancet, BMJ and Annals of Internal Medicine.
NEJM and Lancet have their own versions of weekly audio summaries. You can listen to the MP3 files directly or subscribe via iTunes.
It is a matter of time before almost all major journals feature audio summaries. I will email Brian Mandell of the Cleveland Clinic Journal of Medicine to see if he is interested.
In the meantime, you can always use text-to-speech to convert text from your favorite journal to mp3 files you can listen to on the go.