You just don't trust it. Even if you can't put your finger on it, something's not right. It's, well, sketchy. The sketchy voice is perfect for the character who you just can't trust, who isn't coming fully clean, or just has a hidden motive.
A sketchy voice is just a little off. It's not trustworthy, Urban Dictionary tell us that a sketchy person is someone who acts suspicious, unpredictable and odd in normal situations. You get it, it's the guy who raises your eyebrows or makes you cock your head goind, "hmmm." If this is your character, use a sketchy voice to fully embrace their sketch.
A sketchy voice could be a perfect addition to a video game or there may be a sketchy character in an animated film or audiobook. Your audience is going to doubt and mistrust this character, so only use this voice where you want this reaction.
Often sketchy voices are lower pitched, think Disney villians, Ursula and Scar. Our protagonists trust them even though we are shouting, "Don't!" We already know they are sketchy. They also may sound a bit evasive and condescending. Your female sketchy voice can be high pitched as well as low - both can be considered sketchy. Often, the female sketchy character has that raised pitch at the end of her words, again think of our hated friend, Ursula. That changing pitch lets us know something's off. Stay away!
A sketchy voice is great for a villian or someone with an ulterior motive. Have fun with this one! Make your bad guys come alive.