Typically a mouth sewn shut is a motif more at home in the horror genre, body modification enthusiasts, and more recently as a form of actual political protest as google brought up several pages of such events.
Loki may be the first victim of this practice. Loki had his mouth sewn shut with wire after loosing a bet with some dwarves. He had wagered his head in the bet (which Loki then lost), but refused to let the dwarves take his head if they couldn't remove his head without leaving his neck intact. Instead, the dwarves sewed his mouth shut for his slick way with words. According to wikipedia, this myth is the basis for the logical fallacy "Loki's wager," which "is the unreasonable insistence that a concept cannot be defined, and therefore cannot be discussed."
The next instance is found in Don Quixote, by Miguel de Cervantes. The first installment, published in 1605, is considered one of the world's great masterworks. At one point in the book, Sancho tells Don Quixote, "Senor Don Quixote, give me your worship's blessing and dismissal, for I'd like to go home at once to my wife and children with whom I can at any rate talk and converse as much as I like; for to want me to go through these solitudes day and night and not speak to you when I have a mind is burying me alive. If luck would have it that animals spoke as they did in the days of Guisopete, it would not be so bad, because I could talk to Rocinante about whatever came into my head, and so put up with my ill-fortune; but it is a hard case, and not to be borne with patience, to go seeking adventures all one's life and get nothing but kicks and blanketings, brickbats and punches, and with all this to have to sew up one's mouth without daring to say what is in one's heart, just as if one were dumb."
In 1827, it appears again in the Atheneaum. The piece is titled Painters-Authoresses-Women, but it is not attributed to any author. "It was easier to look in the glass than to make a dull canvas shine like a lucid mirror; and, as to talking, Sir Joshua used to say, a painter should sew up his mouth."
In the Encyclopedia of Superstition, the practice is a symbolic act of protection.
There are also several mentions of the practice in funerary procedures and in the descriptions of the creation of shrunken heads.
Perhaps most interesting, I found a reference in the book The Madonna of the Hills by Arthur Guy Empey, American author of pulp fiction and screenplays.
"..."The other way is to promise him a pardon in a few months' time if he pleads guilty and keeps his mouth shut. Then railroad him to Sing Sing for the limit and I can fix it so he never will be heard from. Do you want to double cross him or are you on the square with him?
"I don't give a damn what happens to him," returned Davis, "just so we can sew up his mouth. The sky's the limit. Croak him if necessary.""
edit: corrected image for the Encyclopedia of Superstition.
(edited 7/31/14 for grammar and tags added)
