There’s a reason the local metal superstars of Lamb of God make the big bucks. Not only is their epic music demanding to play, but have you heard the 40-year-old lead singer’s growling, witch-on-fire vocals? Randy Blythe, that has to hurt. Now, thanks to the modern miracle of iPhone apps, you can stay pretty while sounding like a fried robot version of Blythe using the voice-modulating “Vox of God.” The app, from Epic Records and Appetizer mobile, allows you to tap and record your killer metal lyrics (example: “Mia-wia is the cutest wittle doggie in the world — yes she is”), then play it back either solo or accompanied by an instrumental version of the band’s “Ghost Walking” single. Then share the creation to your favorite social-media time killer and watch those impressed comments roll in. So maybe this $1.99 download is an anticlimactic, one-trick promo pony, and sounds better with headphones. But how many iPhone apps are based on your favorite band’s raging vocals? That’s what we thought.