Random shower thought. Suppose two people are fighting faster than the speed of sound while talking amongst each other, which means they'd also be talking super fast to keep up with their reactions. Would it be possible for the air to "keep up" with their vocal chords to produce coherent speech or would all the vibrations get mumbled together due to being too close to each other as the words smash together. Or perhaps they would have to draw out their words, so instead of saying their special attack "Laser Blast!" he would go, "laaaaaaaaaaaaaaaaaaaa-" for a matter of seconds. And also their breathing might be messed up horribly, like how they'd get the air to draw out all their words. Well, I doubt they'd be hyperventilating during the battle. Technically it would be the same amount of time drawing out from their perspective as it would for people observing, so it would take no more air, just seeming like it's drawn out thanks to their perspective.

Also, I wonder if they'd sound high pitched to outsiders or not even able to be heard.