Let there be music!

What is mariachi music? The traditional mariachi music is what is called the "son." It's a dance tune. That's the traditional music. All mariachis still play that.
Most people know mariachi music through the Canción Ranchera, the Mexican ranch or country song. This is a very passionate genre. It has its origins in the Italian opera - in the bel canto. -Incidentally, Placido Domingo, one of the three great tenors, now sings the canción ranchera with mariachi, so it this genre has come full circle. Placido has recorded a couple of mariachi/canción ranchera CDs.
So that's the mariachi musical tradition (the son and canción ranchera). What is the modern mariachi music? Anything and everything. Different genres. Stuff in English, stuff in Spanish.
There are purists like in any tradition who say "This modern stuff isn't mariachi music." No, if one is a traditionalist but, of course, all traditions evolve. Nothing stays the same. Earlier I noted that mariachis used to be comprised of just four members. Now they have as many as 15.
From my perspective as a sociologist, if mariachis play it, it's mariachi music. In other words, whatever mariachis play is mariachi music.