My new Mac Mini Pro uses HDMI to output sound to a stereo receiver, and the screen mirroring icon shows that mirroring is on. There is no switch to turn it off; I am concerned this may affect performance.
Mac mini
That seems to be how it works in 15.2.
For those that have their Macs connected to a TV to use as the sole monitor, their screen is seemingly mirrored to the TV. But, they don't have another screen to mirror.
Apple hasn't responded to my feedback on the subject, so there isn't much you can do but provide feedback to Apple.
It won't affect anything.
It's an annoying little quirk to an otherwise relatively smooth migration to my new machine.
How to turn off screen mirroring using hdmi for sound output to stereo
