Have the beep repeat again every 15 seconds. Or even 15, 10, 5 (that's 3 beeps in 30 seconds) then 5, 5, 5... At the very least have it beep once more. Another option could be to have it beep again and/or more frequently if no mouse movement is detected. I had no idea it was my turn once (which is my fault, sure, but I play with the TV on, it's not like we are playing a competitive arena game here) and a second beep would have saved the other guy a lot of waiting. Maybe have the color of the screen change completely to a green tint until you move the mouse. That would be pretty hard to miss. Also, sometimes I play with the sound off. I don't always play at home, for example.
