It's an application on try icon that unmute microphone on a keypress
When the Application strat, it will add a status icon. Hold fn key to talk, and release to mute
The icon indicator will be green when talking.
I did not sign the App. To enable exception for this app follow https://support.apple.com/kb/PH11436?locale=en_US
If you want to be sure, just compile the project, Archive & export PushToTalk.app