Well, its not unlogical you can't pass the volume higher then 1. I mean, when the glass is full, it is full! it can;t get more full!
Anyway, if your purpose is to let other players hear the sound harder, then you can try to let the sound emit closer to the player yo uwant to hear it. Because, also the closer the louder the volume right? I bet the engine handles that, so if yo ureally want the max of the max, you should do a 'play' command at that player and play the exact same sound as the player who gets killed.
DO i make sense now?