I found this on Reddit which matches up. It's another SR-71 pilot that says the military aircraft wouldn't even be on the same radio frequency. But military does transit commercial space from time to time so you never know.
Military aviation uses non-military frequencies to communicate with ATC, which they do except when operating in closed airspace. E.g. ATC is notified by the pilot of each aircraft entering the sidewinder low level training route in southern CA.
There's a story of an SR-71 nosing around the Florida/Bahamas/Cuba area only to get a call from ATC asking them to divert due to traffic. At our altitude? So the pressure-suited pilots adjust course for a bunch of French tourists in their jackets and sundresses go past, because the one other plane that flew that high was Concorde.
What I very vaguely remember, from an airshow at Edwards AFB commeorating the 50th anniversary of Chuck Yeager's 1947 flight in the X-1, is that it is clearly audible, but not so window-shakingly loud as when an F-16 went supersonic below 10,000 feet (which they weren't supposed to) where I live. (The latter happened several years ago during Thunderbirds practice.)
I've heard a sonic boom out at Joshua Tree taking pictures at night. I thought a bomb went off until I saw the blur flying across the sky and figured out what it was.
I was visiting with a friend in the desert when Virgin Galactic made it's initial flight into space. I don't know how high they were when they passed overhead, but it certainly audible and rattled the garage door.
Edit: after a quick search, the VSS Unity is released from the carrier at 50k feet. But now I can't recall if the boom was on the ascent or on the way back.
Doesn't Brian Shul talk about the time Regan had them fly figure 8s over some foreign city during negotiations just to remind them with the sonic booms that we can touch them? https://youtu.be/3kIMTJRgyn0?t=1044
Later in that thread someone mentions that the SR-71 has 4 radios (like the story mentioned) 2 UHF, 1 VHF, and 1 HF. So despite not being on the same frequency for most communications, they were definitely capable of monitoring and transmitting on civilian frequencies. A bored radio operator on yet another training flight could easily be listening into civilian radio traffic.
Not an SR-71, but a couple of weeks ago I was listening to a pilot/amateur radio operator making contacts on the 20m band while he flew from Texas to Nevada. I'm assuming their radios are a bit more flexible on what frequencies they can transmit on.
It was a different thread that I remember. Someone posted the story as a submission, and one of the top-level comments was debunking it. My Google-fu used to be extremely good, back when Google was a good search engine, otherwise I would have found it by now and posted the link.
I like the effect that the story has had, but I dislike the idea that it might have been an exaggeration.
The author of Sled Driver made a lot of appearances/talks, but I don't know how much he was paid for them.
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The author states he doesn't normally monitor the frequencies as that was the other person's job, but the one time he does monitor, this happens?
Either things like this happen all of the time, in which case there would be plenty of people sharing their version of these kinds of stories, or the pilot got extremely lucky in his timing.
https://www.reddit.com/r/aviation/comments/xis19w/reddits_be...
But, then again, doing Mach 2.8 in Class A airspace is kind of unrealistic. And noisy.