I've been reading about this bug and I think it may have something to do with the fact that I'm using a child xbox live account, because one of my friends who isn't can still access the realm. I've converted my account to an adult account, but it still doesn't work.
You have to download the vanilla music pack to get the sound!!!!!!
This just happened to me too. One of my experimental gameplay worlds vanished unexpectedly. It is still in my world files, but not visible in the world list. I copied, then compressed the world file, turned into a .mcworld and opened it in Minecraft, but it still didn't appear. I went into the world files and there were two copies.
This also happened to be on Windows 10 1.2.13.
I will post this bug on a separate listing as I'm not sure the information in this comment will be seen.