To play audio from your Mac on TV, the app needs access to JustStream Audio Driver. JustStream Audio Driver is a virtual audio device that captures the output sound and loops it back as an input to Mac. As a result, the JustStream app is able to record your computer’s screen along with system audio and send them as a single stream to the TV. Such kind of loop-backed input macOS recognizes as a microphone and that is why the application requires access to it.

When turning on the sound during the screen mirroring, the user will be asked to access the microphone.

In the invoked window, choose the OK button. If the user does not give the application access to the microphone, the screen mirroring will be done without sound.

If the user denies this request, then the next time when the sound is turned on, another Alert window will be shown.

To give the application access to the microphone, you will need to go to System Preferences > Security & Privacy > Microphone and check the box next to JustStream (see screenshots below).