If you are using Zendesk Talk on MacOS Mojave, you need to allow your browser microphone access in the OS and in the browser itself.
Using MacOS Mojave and Zendesk Talk
Enabling microphone access in Mojave Privacy settings
1) From the Mac System Preferences, click Security & Privacy, and then select the Privacy tab.
2) Select Microphone and then select the applicable application(s) that you want to access your microphone. Make sure your browser(s) are listed/selected