Sound is transferred through a medium literally as a wave. When you get right down to the core of it, the wave requires movement within the medium to transmit.
So it might help to conceptualize it not as "Liquid cannot move faster than the speed of sound in it's medium" but more like "The speed of sound in a liquid medium is defined by the speed at which energy can propagate in that system, which includes motion."
True. You can shoot lasers and such through a lot of things, but I am trying to describe a phenomenon in relatively simple language without getting too bogged down in the technical details. The question was about flowing liquids, so I assumed it was understood that that was what I was talking about.