An easy way to tell if someone is evil is if their voice is much deeper than a good character's, often artificially so. If someone undergoes a Face–Heel Turn, then along with their Evil Makeover, their voice will either drop a few octaves, get put through a voice modulator, or be combined with another voice speaking in a much deeper tone. If an Anti-Hero decides to move a few notches down the Sliding Scale of Idealism Versus Cynicism, then he will start talking in a deeper, more serious tone of voice. Male characters almost always get an unapproachable, commanding cadence. Villainesses usually get a sultry, suggestive one, unless they're supposed to be in command or scary, in which case their cadence will be dismissive and forbidding.
A deep voice makes the character sound strong, mature and competent, so it is a common trait for major villains that the audience is supposed to fear and respect. Consequently, such villains will usually speak in baritone register (or in villainesses' cases a contralto register), more rarely in bass register.
A common inversion is an unnaturally high, often cold voice, and especially laugh. Some characters with an Evil Laugh do high-pitched cackling or be a Giggling Villain, instead of the deeper, traditional one. These are usually a more unnatural and degenerate kind of evil, perhaps overlapping with Eunuchs Are Evil. Taken further, anyone speaking with the Voice of the Legion is incredibly dangerous.
Because deep voices are often considered attractive, this may overlap with Evil Is Sexy. If it's to do with the character having great power it can be related to Power Makes Your Voice Deep.
May be justified if the evil character becomes a huge One-Winged Angel with longer vocal cords, since that would make their voice deeper.
