Created by Gravity Road, the three videos feature Kem Cetinay and Chris Hughes from Love Island, Rylan Clark and Alison Hammond from This Morning, and TV presenters Rochelle Humes and Stephen Mulhern.
They show the six players attempted to say lines including, "Big heads ripping through paper hats," and, "Best time for showing off terrible jumpers," while wearing the mouthpiece, which holds the user’s lips wide open, exposing their full set of teeth.
The campaign was created by Shruti Veeramachineni, with the films directed by Chris Faith.
The videos will be available to watch through Sainsbury’s social channels and on the celebrities’ Instagram, Twitter and Facebook pages. All social content will link to the product page for Speak Out on the Sainsbury’s site.
Laura Boothby, head of broadcast marketing at Sainsbury’s, said: "Our ‘Every bit of Christmas’ campaign celebrates the many ways that the nation lives well during the festive season.
"We hope our customers join in the fun of the campaign by trying out the 'Speak Out' challenge themselves to really get in the Christmas spirit."