I noticed some argument audio on the U.S. Court of Appeals for the Sixth Circuit's website and called the Clerk's Office this morning for details.
Per that call, the Sixth Circuit began posting audio at the end of July and plans to continue, going forward. The court is still deciding how long it will keep the audio up. There was no formal announcement, the Clerk's Office said. A link was simply added to the website.
That leaves only the Second, Tenth, and Eleventh Circuits that do not make argument audio available online. A friendly hint, hint, to those courts.
Find links to the now ten federal appellate courts that post audio here.