2. Click the "Horn" icon with the right mouse button, and select "Custom Animation" in the pop-up shortcut menu. ?
3. In the Multimedia Settings tab of the Custom Animation dialog box, the default "When playing" option is [Pause slide show], and the "Stop playing" option is grayed out. We need to select the [Continue slide show] option, and then we can select the "Stop playing" option. According to your needs, if you want the background music to exist in the "X Slides", just select "X Slides" in the "Stop Playing" option. ?
At this point, you should understand that if you want to add complete background music, you should insert a music file on the first slide and change the "X" after the "Stop Playing" option to the total number of slides.