Sometimes outcomes are regulated, not the means of getting to that outcome.
In your case, murder is illegal, but the instrument could be completely legal. For example, water is perfectly legal, although one could force someone to ingest too much water and killing them. It would still be illegal, even though water is legal.
Counter-exampe: regulations exist relating to bombs, stabbing, and guns, without having any threshhold of maximum allowed pushing force or speed of matter.
Sounds are pressure waves that transmit through the air.
Given enough pressure, those waves could even kill someone (bomb).
If we say that there should be no legal limit to sound pressure, then making a bomb explode and killing people should have no legal consequences.
This makes no sense, so we should put a threshold of maximum allowed sound pressure. The discussion now is where is that threshold.