there are usually two ways. You can add it by selecting the e key.

The method of adding background music and narration is to select the movie box, right-click to open the movie box, adjust the function on the lower left to add background music, and then click on the lower right corner to edit. Or open the E key, select the code box, then right-click to edit, switch to sound, select to play background music, and drag it to the code area.