This is important... The speed of sound varies greatly depending on temperature, altitude, which media it's in, etc. It's doubtful that anything could approach the speed of sound in water.
“The most terrifying force of death comes from the hands of Men who wanted to be left Alone. TRUE TERROR will arrive at these people’s door, and they will cry, scream, and beg for mercy… but it will fall upon the deaf ears of the Men who just wanted to be left alone.”
I'd presume they refer to the speed of sound in air.
Still, faster than that while underwater sounds impossible giving water is some 800x denser than air at sealevel. I do recall some torpedo tech involving torps ejecting a thin film of air around the torpedo body as it moves with the effect being an enormous reduction of friction, enabling a torpedo to move some 200 MPH.
Next challenge after that is to make it still able to be self-guiding to a target.
Yeah, I know about that stuff. It's not possible any more than perpetual motion or FTL travel. Do the math.
Sorry, I guess all this stuff is declassified now.
“The most terrifying force of death comes from the hands of Men who wanted to be left Alone. TRUE TERROR will arrive at these people’s door, and they will cry, scream, and beg for mercy… but it will fall upon the deaf ears of the Men who just wanted to be left alone.”