"Mystery Voice" is the third short based on the Christmas special My Little Pony: Best Gift Ever. It was uploaded to the official Hasbro YouTube channel on December 11, 2018, shortly before being made private, and made public again on December 23, 2018. The animation primarily focuses on Applejack, Rarity, Rainbow Dash, and Fluttershy.
At the School of Friendship, Rarity is in the middle of teaching the Young Six how to make traditional Hearth's Warming dolls. But her lesson keeps getting interrupted by school club/event cancellation announcements over the P.A. system, provided by a variety of announcers. After being interrupted from her teaching one too many times, Rarity goes to complain to the announcers.
Outside the P.A. system room, Rarity runs into Rainbow Dash and Fluttershy, whose classes were also constantly interrupted by the school announcements. The three enter the room and discover all of the different announcer voices were created by a single pony: Applejack. She explains that Twilight Sparkle let her do the school announcements for the day and she decided to make them fun in order to make up for all the school club and event cancellations. Rarity comes up with an idea to put Applejack's voice impersonations to better use.
A short while later, Applejack tells Hearth's Warming stories to the students, using her voice impersonations to make the voices for Princess Platinum, Commander Hurricane, and Chancellor Puddinghead.