I find that there is fairly limited variation in volume with my tin whistles. Especialy at the highest notes. Of caurse if you go for the theird octav, take it outside. Whith my bambo and wood flutes, playing quietly is a thing that took me years to learn. To play a note, the air has to move acrost the flute at the correct speed (my own theory anyway). to play the higher notes softly, you need to get a small amount of air moving fast enough. That takes not only control, but lip strength. you may need to work up to it.