There are quite a few of these hand musicians around...
and have been guests on Prairie Home Companion and
several night time variety shows...though i missed
this until posted on facebook .

My dad used to whistle with a blade of grass between
his thumbs...hands together in similar fashion to this
also the squirting of water that some folks have perfected
while swimming, well, not actually WHILE swimming, as you
need your hands to paddle, but when at a swimming pool or hole...
would probably be a good exercise to prepare the hands for this..