About volume, speaking thinking about the physics side, it’s not about how much open is the aperture. As I said, the diameter of the opening will determine the pressure, so it determines the pitch. Volume probably is connected with the intensity (power, energy per second) of the flow, and this is related to the flow rate. The flow rate is probably regulated by the muscles that support your lungs. If I’m right, you run out of breath quickly playing forte than playing piano simply because you use more volume of air to sustain a forte volume, so it ends quickly.