Paul -

When I played "classical" flute and wanted to practice quietly, I blew gently across the mouthpiece, just loudly enough to get a "whisper." That ought to work for a keyless flute. (My other alternative, fingering the notes very hard on the keys to get slightly-toned key clicks probably wouldn't be as effective - and might give you very bruised fingers!) You can do this on a tin whistle, too, by holding it like a transverse flute and blowing across the hole.

Shy person that I am, I'm an expert on playing quietly. Even bodhran.

PoC