If they're both moving faster than sound, obviously they shouldn't be able to talk to each other.
But have they ever?
And is it ever explained?
Science Fiction & Fantasy Stack Exchange is a question and answer site for science fiction and fantasy enthusiasts. It only takes a minute to sign up.Sign up to join this community
As they move through a medium (air) faster than the speed of sound in that medium, they produce a conical shock wave. Inside the cone, you can hear them; outside it, you can't.
If they are travelling in the same direction, then they can't both be in the other's cone so, at most, one of them can hear the other (assuming they can filter out the noise produced by their own movement).
If they are moving in opposite directions, then they can be inside each others cones and can hear each other, but only sounds produced before they passed each other. In fact they would hear the sounds in reverse as they overtake and pass them (again assuming they could hear the sound through the noise of their own supersonic movement)
I can certainly imagine Superman's 'super hearing' being able to hear through the noise of their movement, and 'hearing fast things while going fast' is certainly the kind of thing the Flash ought to be able to do so the problem of hearing though the noise can probably be dismissed.
So if one is in the wake of the other, then yes, they can hear the other speaking. They won't be able to have a back and forth conversation though.
Of course they have held a conversation while running despite this in the Superman: The Animated Series episode "Speed Demons". It's not explained.
Yes, they can and have done so in canon. And, yes, it has been explained by the writer(s).
In Superman #709, in the "Superman: Grounded" Story Arc, they have what should amount to a 5-minute conversation within the span of a millisecond. During their conversation, "the whole World has just frozen in place" - it's also notable to mention that the entire conversation had also taken up the following 3 or 4 Comic Book pages of dialogue, as well. Ironically, this conversation brings up the things Flash, while moving faster than sound, spoke to Superman earlier in the comic to get his attention.
Here are the scans:
In these scans, you can see that the the background is essentially frozen throughout their conversation. The woman who spilled the coffee (and the coffee itself) in the first scan stays frozen through most panels and shows minimal movement at the end of the conversation when the Flash intervenes (it is unclear whether this is due to elapsed time or the Flash's intervention). Also, the kids seen looking at Superman and Flash (in the booth behind the Flash) in Panel 2 of Scan 1 are shown in the same position in Panel 4 of Scan 3 and with minimal (if any) movement in Panel 1 of Scan 4.
Here is a scan of Superman using his super-hearing to pinpoint Flash, who was speaking Kryptonian at hyperspeed and moving faster than sound (Superman #709):
Also, the writer of Superman #709, Chris Roberson, had some interesting things to say, in regards Superman having to "speed up" his senses while using his Super-Speed. Not only does this interview actually confirm that Superman has massively beyond nanosecond-precise perception and reaction time - even up towards attosecond-precise, according to Roberson himself - but also brings out that there actually isn't any drawn out speed-up time for Super-Speed perceptions and reactions:
"It's a fair question, to be sure. I wasn't thinking of Superman "activating" his superspeed perceptions, like it was a light switch that could be turned on and off, so much as it was shifting his perceptions to bring superspeed events and sounds into focus. Like the human eye looking at something small and close up and then adjusting when the viewer looks at something large and far away, I think Superman is perceiving all of these things, ALL of the time, and it's just a question of what he chooses to focus on. But I think Superman is also capable of focusing on macro-scale, "normal" speed events at a rate something similar to that in the typical human range, so that he wouldn't spend a subjective eternity between each word in every sentence that Lois says to him. Similarly, he can chose to perceive visual information much like we do, or he can expand his perceptions and see far into the electromagnetic spectrum, or narrow his focus far enough to see individual atoms. But just like he doesn't see Lois as merely a cloud of electrons, neutrons, and protons, he doesn't perceive every second as composed of a huge number of attosecond-scale events--unless he wants to, of course."
But that's just one fan's opinion, too!
Best, Chris Roberson"
Of course, this doesn't make physical sense as sound moves at 343 m/s in air and therefore should necessitate a longer conversation time, but c'mon guys it's comics. Some physical laws have to be suspended.
Extra: Here is a scan of Barry Allen talking to Wally West while moving near lightspeed, which is WAY faster than speed of sound (Also notice that Superman is able to perceive them):