the playsound command on command block does not work in mulyiplayer when there is a @a

when I put /playsound mob.enderdragon.end @a in the command block and there was more then one player on a server then the command block will not play
but the command block will play on single player when there is only a single player on the server.

How to recreate
put in the /playsound mob.enderdragon.end @a in a command block when there is more then one player on the server.