Before adding audio to the tour ensure you have uploaded it in the "Media" tab, a guide for this is located on the help guide page
Go to the "POI Editor" tab on the left side of the screen, then find the panorama you wish to add audio button to and rotate the panorama to where you want it to be placed.
Drag the arrow icon at the top middle of the screen (shown below) to where you want the audio button to be placed.
A pop up menu should appear on your screen after placing the arrow icon
From the menu, choose "Audio 3D", then select the audio file you want to be played when the button is clicked.
Once the button has been made, click on it in the editor and another menu will appear. Here, change the button type at the bottom to "button". Here you can also choose whether to have your audio loop, its volume and its range. Changing the audio's range will increase/decrease its volume when the user is turned away from the button.