How to click a label, or area of UI, to select group?

A switch or button would be more appropriate for this action.

Please read what you posted "...and by clicking on one a specific group gets selected to be able to sound. The simplest way to get started I think is to just use labels."

It is hard to understand what you want. I will guess:

You want a number of buttons on the UI and when any of them are pressed, all the others will be turned off (not pressed) and that pressed button will point to the group you want to play.

If that's what you want:

on init
  declare ui_button $button1
  declare ui_button $button2
  declare ui_button $button3
  declare ui_button $button4

  declare %button_uiids[4]
    %button_uiids[0] := get_ui_id($button1)
    %button_uiids[1] := get_ui_id($button2)
    %button_uiids[2] := get_ui_id($button3)
    %button_uiids[3] := get_ui_id($button4)

  declare $i
  declare $group_to_allow
end on


function clear_buttons
   $i := 0
   while($i < num_elements(%button_uiids))
      set_control_par(%button_uiids[$i], $CONTROL_PAR_VALUE, 0)
      inc($i)
   end while
end function


on ui_control ($button1)
 call clear_buttons
 $button1 := 1
end on

on ui_control ($button2)
 call clear_buttons
 $button2 := 1
end on

on ui_control ($button3)
 call clear_buttons
 $button3 := 1
end on

on ui_control ($button4)
 call clear_buttons
 $button4 := 1
end on


on note
  disallow_group($ALL_GROUPS)
  $i := 0 
  while ($i < num_elements(%button_uiids))
    if ( get_control_par(%button_uiids[$i],$CONTROL_PAR_VALUE) = 1)
      $group_to_allow := $i
    end if
    inc($i)
  end while
  allow_group($group_to_allow)
end on

 good ol' vanilla KSP.