I had a tough time with this too, especially since i think some recent update modified this (May/June 2020). To turn off the sounds in MS 365 products (say PowerPoint; I'm on a Mac):
Click on Powerpoint (or the name of the Application, in bold, top L of screen)
Click "Preferences"
Click "General" (top left of window)
Under "Settings" unclick "Provide Feedback with Sound"