To add sound, go to the "Sound" tab in the toolbar menu and:
Click "Add music" to upload your own music (we recommend using Creative Commons licensed music to avoid Copyright issues).
Alternatively, select from the library of music available in the "Free Music" section.
When you select the music you would like to use, you will need to select the option to add sound to the "Entire Powtoon" since this is the free version only. This music will then automatically adjust to the length of your project.