I posted this in the General topic, because the solution posted here (hopefully!) will work for every MIUI based device, I suppose.
Any ideas for a file that I could edit?
go to /system/etc/bluetooth/
find audio.conf and edit it
go to the line starting with "Enable"" and change it to "Enable=Sink, Control, Headset, Gateway Source"
go one line below, which starts with "Disable=" and delete everything in that line following the "=" sign
This doesn't work, unfortunately.