I did a tiny bit of troubleshooting by verifying that the issue occurs with multiple apps (Safari/Netflix and Music on my MacBook Pro), but only one device.
I took mine to the Genius Bar today, because I got them through a back-to-school promotion from Apple last year. We verified that the problem persists when used with a “known good” source. He asked whether I may have over-extended that side of the set, but I hadn’t even ever extended it all the way (I usually only extend it one ‘click’ from fully closed).
The other cables on that side (power and bluetooth) both work. The only sympton is no audio on that side.
“Repair” options were to send it out—they don’t deal with them in-store—which would take a few days and cost $129, or to buy a new pair for $199. Neither are good choices for me to replace a ‘gift with purchase.’
I guess I’m going to have to take them apart. Since there’s no guide here (why I came to this site), I’ll just have to wing it. I’ll let you know if I figure something out.