nx-btrednx-btred is a Bluetooth audio driver/redirector for Switch.
It uses the audrec:u service to record game audio, and then outputs it on the new audio bluetooth driver introduced in firmware version 12.0.0.
YOU USE THIS AT YOUR OWN RISK. Not every headphone brand has been tested. There may be really loud noise. Be careful, start out with a low volume, and then increase, if you like. In case of any type of malfunction, the sound will stop immediately if you hit the power button.
Install firmware 12.0.0+.
Install latest Atmosphere.
Install latest MissionControl.
Download nx-btred and unzip to your SD card.
Enter the homebrew menu.
Launch the btpair application.
Press X to scan.
Select your headphones and click A.
Wait for it to pair.
Due to a limitation of the audrec:u service, only games audio can be recorded (not the system applets).
Thanks to ndeadly, SciresM, yellows8 and the rest of the Switchbrew crowd