1. Either create a CT_LISTNBOX_CHECKABLE or use the multiplayer player list on the left hand side of the map.
2. Add CheckBoxesSelChanged event to control.

_playerList = (findDisplay 12 displayCtrl 1002);
_playerList ctrlAddEventHandler ["CheckBoxesSelChanged",{
	systemChat str _this;
}]

1. Click the checkbox on the control
2. Move the mouse slightly and select the same checkbox
3. See difference in second return index

Was trying to figure out a way to check if a player has a different player muted and came across this issue, as you cannot select the proper index of checkbox due to syntax limitations of ctrlChecked, but can't add that without this being fixed