This script is by JulioNIB, he of the awesome Incredible Hulk mod. (Skip ahead about 45 seconds to see it working). Unlike Saints Row IV, dubstep does not appear to be lethal with this mod—it just makes people dance. But check it out when you start blasting cars! Goddamn, that's funny.
There are three firing modes, standard, a spread-fire, and a grenade mode with splash dubstep damage, so Niko may go about (not) killing them harshly with his song. If you're looking for the real thing, Saints Row IV arrives Aug. 20.