To add audio (you will need to have it be an MP3) you can use a website such as 123apps' online voice recorder. What is great about online voice recorder is the fact that it allows you to trim your audio, save it, and the fact it is free.
You can also use audio from your phone (and upload it to a site such as Google Drive) and then convert it to MP3 using 123apps' online audio converter.
You can then add the audio recordings directly in your tour by clicking on the area (or scene) you would like to add audio and click on what you see below (looks like a person with audio). The other option is to add ambient noise.
Once you upload your audio, publish your tour, and share it - please remind your viewers to select the gear (upper right) and make sure they click on the gear and turn narration on.
Embedded below is a 2 minute and 25 second YouTube tutorial I created.