The notification sounds should be the same for all chat notifications regardless of if they are internal or external.
Could this following scenario apply? Just taking a stab here...
If the current chat is active and in focus, the sound will not occur. For example, if I'm chatting in a room with you and we're talking back and forth, I won't hear a sound while that room is open. Only if I have some other chat/view open will I hear a sound alerting me to activity in a room/chat that is not currently in focus.