One possible way: Atoms, like the Solar System, are mostly empty space.
If a craft can be “tuned” to simulate dark matter, neutrinos or some other non-interactive substance, it would move literally through ordinary matter air without disturbing it.
In other words, the craft would slip through the air without the air interacting with any part of the ship.
No interaction, no shoving the air out of the way, no shock wave, no sonic boom.
Still, any way you slice it, “if the observers were honest about what they witnessed, and I know I’m being honest about what I saw and heard, and if the craft observed really did what they claimed it did and left no sonic booms...” you’re talking about a technology existing that isn’t out there for general dissemination yet. OR, “they” have figured out how to manipulate phasing characteristics to also allow living breathing beings to pass through mediums and survive while piloting these vehicles.
It all adds up to something “other-worldly” going on if the event the witnesses describe truly occured.