I've had the black update since Thursday(1.30.14), and I only experienced the volume issue once, about 6 hours ago.
BUT, I did have a similar issue, but it was only when listening to podcasts. Once started the volume wouldn't change from the level it was at when it was started. The only fix was either play something else then go back.
as for what happened earlier, I was using the Xbox music beta, streaming with the radio feature(smart DJ) earphones in(wired), stopped it and removed the earphones. A few hours later, I clicked the play button it was too loud (30/30) attempted to turn it down (0/30) with no change.