After I used the Mirror to TV app, I lost the sound on my Mac
This happens when you quit the application before you stop the mirroring. The correct way to stop mirroring is to first click on “Stop Mirroring”. Then, if you need to, you can also quit the app. If you quit the app before stopping the mirroring, the audio driver selected will be AirBeamTV, not the internal speakers.
When you have quit the app before stopping the mirroring and the sound is no longer coming from the Mac, you can always go to System Preferences / Sound and select “internal speakers” as your preferred sound output.