Currently, the only way to play sounds for only the avatar wearing an object is to use llPlaySound in an object attached to the users HUD.
Sometimes, a creator might want to play a sound only audible to the avatar wearing a particular non-HUD object. The only way to achieve this currently is by force attaching a separate HUD-object using RLV. Without RLV it is impossible.
A function along the lines of llPlaySoundForWearer and/or llPlaySoundForOwner would be a great addition.