I am having a similar issue. I have an S8 Active and a Samsung 4K SmartTV When I project my phone to the tv, I get good video, but no sound whatsoever. I have switched the audio output from TV to phone, and can get sound through the phone but never the TV. Did you ever get a solution?
There are two things that come to mind. Trying going into
1. Advanced sound settings
2. Separate app sound
3. Choose which app you'd like the sound to play through the device selected.
Otherwise, you can try this if you're using a USB connection
2. Developer Options
3. (Media) Disable USB audio routing
Hope this was helpful!
Be sure to click " ✓ Accept as Solution" when you find an answer that works for you.