When watching a plane flying over Mach1 in fly-by mode, I would except the following:
1. No sound heard when plane is approaching
2. A boom sound at some point as it's passing
3. Full sound as it passes.
Instead, I hear:
1. Full sound heard when plane is approaching
2. A boom sound at some point as it's passing
3. No sound as it passes.
Also, while orbiting around the plane over Mach 1 in external view, I wouldn't expect to hear any sound while situated in front of the plane.