These brakes are not air brakes, nor are they air over hydraulic brakes. You will never, ever get that kind of sound from them. If you do, you should fix that immediately.
The air assist mechanism does vent air every time you release the brake pedal. It vents out a long tube, away from you, and in WAY smaller volume than an air/air over system. It is not loud at all, and does not carry well over the noise of a running multifuel engine.
If you have assisted braking (try with the tanks open or low for comparison, on flat ground with lots of stopping room) then the air pack is receiving air at the right time.
If the brakes release promptly then the air pack is venting properly. Hold yourself on a hill, in neutral, then just let go of the brake pedal. Look for an instant release of the actual brakes (not the pedal) and not a delayed release. A failure to vent will cause the brakes to remain applied if it's a full failure, or a partial blockage (slow release of air) will cause a slow release of the service brakes.