I got Samsung to help me out with this issue. All I had to do was unplug the TV from the power outlet, wait 3 mins and then plug it back in. It solved the screen mirror audio issue.
As I mentioned above, Samsung fixed this issue for me.
All you need to do is turn off your tv, unplug it from the wall, wait 3 - 5 mins, plug it back in and turn on your tv. A reset of sorts. You should be able to have audio when you screen mirror now.
Ok everyone I think I got it figured out. Go to your normal SETTINGS on device. Then go to SOUND settings. Click on the VOLUME tab. While you have your device connected to your TV using smart view turn up the MEDIA bar all or close to all the way. That fixed it for me after months of trying to figure it out and giving up multiple times Haha. Hope this works for everyone else. Feel stupid for how simple it ended up being.