Sollux is usually a sarcastic, unbearable troll with a lisp. If you don't get what I mean, just look at the two lines I put down! But, he tends to switch from being insufferable to being somewhat calm, less sarcastic, and more receptive and mindful of things (and trolls). That's when things aren't about to GO TO SHIT, or when Karkat flips HIS SHIT, or when he's dead/half-dead, like in line three. His voice would have his lisp, include sass for his remarks, and hints of (if not, downright) sarcasm. Except for when he's dead/half-dead, basically when he uses Aradia's quirk.