What that got to do with measuring the speed of a plane ?

The speed of sound is 767 MPH.
The speed of the plane was 745 MPH.
The plane is travelling 22 MPH slower than the speed of sound in still air.

Now if you want to start talking about the speed of sound in or around the plane, that's another matter.

There's a difference between something "breaking the sound barrier" and something travelling at a comparable speed that sound travels at.

A bullet fired from a moving gun might be travelling faster than the speed of sound, but it might not "break the sound barrier" depending on how fast the gun is moving and how fast the bullet came out of it.