I don't think this will reduce accidents at all. Most cars I get into already ring if the driver's belt isn't buckled. Some cars have a speed cap built into the ECU that force them to stop accelerating at a certain speed.
They say 5k teens die from car crashes each year, but they don't say how many of the 5k die because of reasons that this key can prevent. My guess is it's not that high. This key can't prevent a stupid decision or a silly stunt.
I'm all for the speaker thing, though. I get into my friends cars all the time, and they immediately turn up the radio all the way. They think it's cool, but working for a sound productions company teaches you better. The sound that comes out of a maxed out radio (most, at least) is nothing but garble. It gets even better when they max out the bass, and you hear this horrid braahhhmm every time the speaker beats. It just sounds terrible, I don't know why people do it.