I can't enable Audio either. This has nothing to do with phone calls because audio works fine with the calls. The Audio setting is for the PC to receive audio for apps running on the phone. Normally it has no way to do that.
I first realized the audio problem when I used Phone Link to try to play a game on my phone. The game appeared OK on my PC screen but the audio was was still coming out of the phone. The solution was an app I downloaded from the Microsoft store called Bluetooth Audio Receiver.
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If I ran the app and Connected to my Samsung S21, the audio for the game started coming out of my PC speakers. Also, when I did this the Bluetooth Audio setting on my phone was turned on. If I disconnected the Bluetooth Audio setting on my phone was turned off again.
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