My issue is slightly different. I recently switched phones from an S8 Edge to an S10 Edge.
With my S8, I could mirror screen on my Samsung TV and then choose to play the sound through my phone.
Now, with my S10, I can do one or the other, but not both simultaneously. If I'm mirroring my screen and then ask for audio to play on my phone, it switches off the mirroring and enables audio on the phone. Never both.
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