There isn't currently any kind of volume control; volumes are effectively "baked into" the amplitude of a sound's samples.
A slightly clumsy workaround would be to make a copy of an existing sound on the fly and use sound.map[] to rescale its samples. Assuming the deck contains a sound clip named "sosumi":
on play_scaled name vol do r:-128+range 256 play[sound[deck.sounds[name].encoded].map[r dict floor vol*r]] end on click do play_scaled["sosumi" 1] # normal volume sleep["play"] play_scaled["sosumi" .5] # half volume sleep["play"] play_scaled["sosumi" .25] # quarter volume sleep["play"] play_scaled["sosumi" .10] # 1/10th volume end