How to find the source of unwanted Audio on iMac?

I have an iMac and today I lost electricity and all shut down. When my browsers opened again some videos and news channels all started playing. I methodically closed them but I then heard some people talking and coughing and walking. It is the same as a guy reported here back in 2013. It lasted for about 15 minutes then it disappeared. Does anyone know what this could be? I checked that there is no remote connected. Thanks in advance.

[Re-Titled by Moderator]