When I typed /playsound mob.endermen.scream and then pressed tab, the command did not come up with my player name. Most commands which have a target player or entity argument will fill in my player name when pressing tab to autocomplete, for example:

/give (press tab) enters my player name
/entitydata (press tab) enters my player name
but /playsound <sound> (press tab) does not come up with anything. It still does not when I type my username halfway and press tab.