What the Problem is
This is most likely a chunk glitch, basically the game froze up somewhat and didn't render in a chunk. This usually happens when in Creative or Spectator because you are moving much faster than you can in survival.
It's more common on multiplayer than single player because the server (even if you're connected via LAN, it's still a server) has to do more and there's way more chance for the server and client to get out of sync.
There are a few things you can do.
Quickest/easiest thing to do, would to press F3+A. This will force the game to reload all of the chunks and should fix this visual glitch.
If that doesn't fix it, try leaving the game and coming back in - that will totally reload all the chunks.
As for why you got stuck, you probably flew into where the server knew there were blocks, and then your client halfway caught up.
The server knew where you were and that you were in a block(s) and so told your client that you couldn't move, expecting your client to know this from information that it should have received earlier about your location/that chunk.
However, since your client most likely didn't receive that information earlier, it just stops you from moving.
As for the music, that's just part of the game - music randomly selected from a list at certain times of the day. See here for more information.