In the episode
Secrets of Superboy (1990), Superboy was able to detect a subliminal message with his super-hearing. In this one, the only way he figures it out is noticing that everyone starts acting normally again after a TV is turned off. Considering that it was affecting people even when they weren't looking at the screen, it would have to be a subliminal audio message that is affecting everyone, meaning that Superboy should have been able to pick it up as he did previously.