Planes are stress tested well beyond the specs that are recommended or set as max operating specs. Just look at the wing deflection test
With a tail wind, the plane may be moving faster than the standard speed of sound, but the air that they're moving through, in a relative manner, is slower than the point at which a sonic boom shockwave will form. You have the speed of sound that changes based on altitude and atmospheric pressure. You also have measurements of indicated air speed versus true air speed.
There's a reason that aerospace engineering is usually a masters-level program.