Put the summary of the bug you're having here
When I use @a for the playsound command on multiplayer, it does not play the sound. When I changed @a to @p or @r, it worked fine.
What I expected to happen was...:
It would play the specified sound to every player.

What actually happened was...:
It did nothing.

Steps to Reproduce:
1. Go on a multiplayer server with at least one other person.
2. Open a command block and type "playsound mob.enderdragon.end @a
3. Power the command block with redstone.