There are two ways to add audio. In one, you create a slide that contains only audio. In the other, you can have audio alongside other elements.
In order to create an audio slide, go to add new slide, add content. At this pop-up click on the arrow to see more content. Select Audio.
Next, simply drag and drop it onto the pop up or select from where you would like to upload the audio. It can be from your local files, Google Drive, Dropbox, box or OneDrive. Once you have finished, Save. A new slide containing audio will be added to your lesson.
If you would like to add audio to an existing slide that contains other elements, double click the desired non-activity based slide. In the bottom-left corner you will see the options layout, audio and preview. Select Audio.
From there, you can either select Audio File or Audio Recorder as shown above.