This problem usually pops up for a few reasons:
Using headphones or external speakers?
Using an older PC computer that uses external audio
Do you use headphones?
If so, the most likely issue is that your computer is confused by whether it should be using your headphones or computer speakers.
Plug your headphones into the computer
Refresh the page/classroom and make sure you select the headset (unless there is only one option)
Did that work?
Do you use a PC with an external audio driver/speakers?
For example, we've found some Asus computers have an external audio driver that needs to be checked and sometime reinstalled.
Are you taking a class at work or Uni where there may be a firewall or do you have a personal firewall?
Click here if you don't know what a firewall is and if you think there might be a firewall issue, check out this article.
If none of the above help...
If these aren't working, you'd be surprised how often a computer restart or at least a browser restart works.