Control volume in android application delphi xe 7

I'm using FormKeyup procedure to on/off sound volume, like this:

if key = vkVolumeDown then begin
quadrum:= 0.2;
 if MediaPlayer1.Volume - quadrum > 0 then
  MediaPlayer1.Volume:= MediaPlayer1.Volume-quadrum;
 if MediaPlayer1.Volume - quadrum < 0 then
  MediaPlayer1.Volume:= MediaPlayer1.Volume.Minvalue;
 snd:= false;
end;

But it's working only to switch on/off the sound (plus-minus 0.2 doesnt work) and i just want to set