Amy's voice-over cameo appears during Young Sheldon season 5, episode 16, "A Suitcase Full of Cash and a Yellow Clown Car." She and Sheldon talk about a few things that affect a relationship. At one point, Sheldon asks if she's jealous of what he's doing, referring to narrating the events of The Big Bang Theory spin-off. Amy doesn't answer his question directly but reveals that they have a microphone in front of them. This means that Sheldon's voice-overs are for something more official; it isn't just him casually recalling his childhood. This further supports the theory that Sheldon is actually in the process of working on his autobiography. Based on his latest conversation with Amy, she isn't a part of it, so it might not be something that's born out of their joint Nobel Prize in Physics win. Instead, it could just be a book deal specifically for Sheldon.
An earlier Simon Helberg (Howard) cameo on Young Sheldon previously reinforced this hypothesis. During the pair's bickering, Howard revealed that he was merely asked by Sheldon to come up with an introduction to engineering for the story.