Netherrack, paranormal events, and C418-11: prank, mod, expected?

Someone went to ... some lengths to pull that off. A jukebox in Minecraft can be heard from up to ~60 blocks away (while the Wiki says approximately 65, I've found it to be about 60). So, this is what they probably did:

1. Placed a jukebox far enough away that you didn't notice them, but heard the song, then inserted the disc.
2. Had a second copy of c418-11 and a piece of smooth stone
3. Managed to sneak by you using an extended potion of invisibility (brewed with redstone to increase duration)
4. Placed the stone block and punched it into cobble in front of you, after dropping the second copy of the disc
5. Sprinted back out of sight before the potion wore off, picked up the jukebox

You got lucky, if they managed to sneak a piece of smooth stone, they could have just as easily had a lava bucket, or a spawn egg that would have been quite difficult to deal with.

If you're going to be playing on a server where quite a bit of fun can be had in pranks, I recommend installing SecurityCraft. It's got (among insanely over-powered land mines) these cool portable radars that can alert you to another player coming into a predefined proximity, fake water (which is actually lava) and fake lava (which is actually water). There's also better doors and locking mechanisms to keep prying eyes out of your stuff.

Anyway, my point - it could have been just another player and not an admin / op of the server, just using things available in vanilla. Hence, anyone with a brewing stand is a suspect (or those known to associate with those potion-brewing pranksters!)

An admin mod or owner played the music disk and trolled you. If you were on a low server it was probably the owner and if it is a popular server it was probably an admin.