I don't know if anyone in the show can actually see it or not but sonic scream has always been visually treated that way in comics, cartoons and TV (even Black Canary's canary cry has the same visual effect). At very least it's a visual cue to us that the power is in use. Again, not sure if the characters see it or not, don't recall it ever being addressed.
Are you serious?! You can see Silver Banshee's scream. And even if you or I can't see it, Clark SHOULD be able to see the vibration in the air caused by the scream and be able to use his enhance auditory ability to tell when it's coming.
