First, check to see if Webex is up and running properly. You can check https://status.webex.com to get the latest status. Or, on Webex for Windows and Mac, you can also use the Health Checker within the app itself.
If you're on a home network, make sure that other devices aren't tying up bandwidth. Major culprits for bandwidth usage are video streaming, online gaming, and Internet calls. Other devices may be occupying the bandwidth or your connection may not meet the minimum requirements to run the app. The internet connection also may not meet the minimum bandwidth requirements.
From the same network that you're trying to use the app on, run the Cisco Webex Network Test in Firefox or Chrome on your computer. Copy the results to your clipboard and paste them into a text file in case you need to provide them to the support team.
If you're still having trouble with audio and video in Webex, you can contact contact your company's support team, or open a support case for Webex. Attach the text file with the results from the network test.