Probably your microphone is not properly connected to the Webphone. You can check this in the settings. Under the microphone, there is a bar. If it does not turn red, you know that your microphone is not working.
When using the Webphone in a Citrix (secured) environment, the microphone may not be allowed to be used. This is likely the case here. It's best to ask your IT specialist to check the settings of this environment to see if it's still possible to use the microphone..