Princess Peach's voice
From the Super Mario Wiki
“Listen to me, everyone!!! Something terrible has happened! The Goodwill Ambassador from our neighbor, the Beanbean Kingdom, just paid a visit! But it was no friendly visit! The ambassador stole Princess Peach's voice!”
Princess Peach's voice is a major plot element in Mario & Luigi: Superstar Saga. Because Princess Peach has a pure soul, her voice is able to awaken the Beanstar, which Cackletta and Fawful plan to use to take over the world. At the beginning of the game, Cackletta and Fawful appear to steal Princess Peach's voice, forcing Mario and Luigi to travel to the Beanbean Kingdom to retrieve it. It was also heard in other games like Super Mario 64 and Mario Kart 8 even though it was more important in Superstar Saga.
In the game's opening cutscene, Cackletta and Fawful disguise themselves as goodwill ambassadors and steal Princess Peach's voice, causing Peach to faint and the villains to head back to the Beanbean Kingdom.
Mario and Luigi are informed of what happened and soon arrive at the castle, where they see that Peach's speech is replaced with explosives (that fall from her speech bubble and detonate). In order to retrieve the voice, Mario and Luigi head to the Beanbean Kingdom, eventually making their way to Beanbean Castle. There, Lady Lima tells Mario and Luigi explains that the Beanstar, an artifact that has the power to grant all desires, was cast into a deep sleep to prevent it from being used for evil. This sleep could only be broken by the voice of a pure soul, and Princess Peach is currently the only one to have such a voice.
With a tip-off from Prince Peasley, Mario and Luigi travel to Woohoo Hooniversity where Cackletta and Fawful are believed to be using Peach's voice to awaken the Beanstar. Within the Hooniversity, they spot Fawful making some final adjustments to one of three Peach-bots positioned around the Beanstar, which are programmed to play a perfect copy of the stolen voice. However, the sound they make enrages the Beanstar and causes it to break through the floor. One of the Peach-bots falls into the basement with it and makes the sound of the stolen voice again, causing the Beanstar to fly out of the building and break into pieces.
Mario and Luigi are later informed of the arrival of Princess Peach, who has come to visit the kingdom, and when they meet with her, they are surprised to hear her speaking with her normal voice. Toadsworth reveals that Princess Peach's voice was never stolen, since Prince Peasley got wind of Cackletta's plan and beat her to Peach's Castle to warn the princess. They then got Birdo to masquerade as Peach and meet with the "Goodwill Ambassador" while the real Princess Peach and Toadsworth hid behind a curtain. Therefore, the voice that got stolen was Birdo's voice.