Adding the CheckBoxesSelChanged event on CT_LISTNBOX_CHECKABLE control type will always return the second parameter different than documented in
onCheckBoxesSelChanged
Using the default map player list as an example, adding the aforementioned event to the list, then clicking on the speaker icon to the right of the player name will have a result of
[Control #1002, 31.7809, 1]
Where the second index should be "_selectedIndex", it is seemingly something to do with vertical mouse position as it will change when moving the mouse even in the same selected index.