Sudden Fear (1952)

Wealthy playwright Myra Hudson is the heiress of a great fortune; however, she works and is donating part of her inheritance to foundations. When she watches the rehearsal of her play, she asks the director to replace the lead actor Lester Blaine, believing him inadequate for the lead role (which, after all, she created). When she returns home, she meets Blane in the same train and they travel together. They stop in Chicago and soon Myra is seduced by him. They get married and live at Myra's home in San Francisco. Myra summons her lawyer Steve Kearney to change her will and transfer her fortune and properties to her beloved husband. She uses her Dictaphone to record the changes to be done in her will. Then Steve leaves the room and prepares to travel to Sacramento with his son Junior; immediately Blaine and Irene Neves, Junior's girlfriend and Blaine's lover, come into the room to plot a scheme to kill Myra so that he will be the heir of her fortune. The next morning, Myra learns that she has forgotten her Dictaphone and left it on, and when she's about to dictate her new will she hears her husband's conversation with Irene.

IMDb: https://www.imdb.com/title/tt0045205