There are quiet whistles about.

I'd hesitate to agree about the Clark Original. They take a lot of air, and I've often found them loud enough for sessions, although, that may vary between individual whistles. Clarke Sweetones and Megs are a bit quieter.

Tony Dixon makes a nice quiet ABS whistle called the SV or "Soft Voice". It's a nice sweet sounding whistle made for quiet practice. Available from Phil at Bigwhistle.co.uk. I can recommend that as a practice whistle from experience - I have one.

The Alba Q1 and the Black Diamond are also quiet whistles, but much pricier than the Dixon.