The Playsound command seems to be broken in prerelease 4. I used the /help playsound command to get the syntax for the playsound command. I entered the command, which worked before and is present in the autocomplete list. The player @a should also work just fine.

I, however get "An unknown error occurred while attempting to perform the command" as an error message.

EDIT: The syntax changed, but is not present in the lang file of my resource pack