But the advantage is everyone on the ground knows where you are.
I would assume not. After all, it was a spyplane. Would be a bit daft to give your position away by sound alone, would it?
But can you hear a sonic boom generated at FL600 or higher?
More SR-71 anecdotes:
<Garbled callsign>: Request FL600
<ATC>: Well, XXX, if you can get that high, you can have it.
<SR71, a bit miffed>: We are descending towards FL600, sir.
And of course, this classic:
http://fromtheinside.us/thinking/Groundspeed_Check.htm