Here's what finally solved my issue:
https://discussions.apple.com/thread/5756897?answerId=24436903022#24436903022
From this Apple Community User:
https://discussions.apple.com/profile/Winston%20Churchill
Try the following steps, check whether things are working after each step where appropriate, before trying the next.
- Restart the Apple TV (Settings > General > Restart).
- Restart the Apple TV by removing ALL the cables for 30 seconds.
- Restart your router. (Also try removing it’s power cord for at least 30 seconds)
- Reset the Apple TV (Settings > General > Reset > Reset all settings)
- Restore the Apple TV (Settings > General > Reset > Restore)
Apparently there is an issue if you push the Apple Remote before you set your computer to mirror. Whatever I did, it had locked it in that state. Once I stopped mirroring, then went onto the TV screen and restarted the Apple TV (#1 above) it then allowed me to choose mirroring on my Mac and the audio automatically switched from the computer to the TV (making the computer now mute), BTW the volume needs to be maxed on the computer as that will transmit to the TV if your volume is low on the computer. Then you can raise or lower it the way you want on your TV.
Yay!
Hope this helps others.....