Chrome:
In Chrome, click on the padlock icon in the address bar, and go to 'Site settings':
Then, ensure 'Sound' is set to 'Allow':
Firefox:
With Firefox open go to:
Firefox
Preferences
Privacy & Security
Click 'Settings...' next to Autoplay:
Then set the default to 'Allow Audio and Video' or define a custom setting for 'class.lingco.io':
Safari:
Ensure you have class.lingco.io open and go to:
Safari
Preferences
Websites
Auto-Play
Then, ensure 'class.lingco.io' is set to 'Allow All Auto-Play':