I suspect that the reason that certain types of martial arts bring about a less violent response to stimulation in the practitioner is that martial arts teach self control. I have noticed, in adults too but especially in children, that their perception that they cannot control themselves leads to their rapid loss of self control.
Maybe violent lyrics, on the other hand, give permission to those normally in control to just let go? I have not actually seen that demonstrated.
We do know that modern society believes whole heartedly in advertising, and I can't imagine violence being more heavily advertised than on current television and movies and videos.