Can't resist throwing my $0.02 into the pot...

I've been playing whistles (C, D, G, and Low D) for about 6 years and and orchestral flute (one student-quality closed-hole Gemeinhardt) for about 3 years and recorders (wood and PVC Sopranos and Altos, wood tenor, PVC bass) for about 20 years.

My experience has been that on the fipple instruments (recorders/whistles) only VERY minor changes in volume can be controlled with air flow. The way to get more volume is to use an instrument with a larger bore. For example, Yamaha plastic soprano and alto recorders are good and loud (as compared with some that are loud but not good!). This also holds true for pennywhistles--the larger the bore, the more volume.

The transverse instruments (eg orchestral and bamboo flutes) have a more dynamic range. The more air you push across them, the louder they get. If the pitch shifts sharp or flat because of the increased or decreased airflow, the embouchure can be modified to correct it.

Hope that helps...your mileage may vary...

V