You can either select 'automatically' or 'manually'. Select 'Effect Options'. Click on the 'Audio' button to open the drop-down menu and then select Audio on my PC.
To play these video files within PowerPoint, set playback options in your presentation file. Reliable and efficient delivery, as fast as 12 hours. You can preview them and select the one you want.
You can either use the YouTube search bar in the box and search for your video, or go with the second option and paste the embed code of your YouTube video there.
With the audio icon selected, you can go on the 'Playback' tab and select 'Play in Background'. A pop-up window will open, asking you how you want the sound to start in your slideshow. On the slide on which you've placed the movie, click on the file to select it.