If you turn on Sonic or Atmos for Headphones, it will switch the Default Format to a 2ch format, usually without warning (which is a problem in my opinion). Sometimes you have to close and open the options to see that it switched to 2ch if you haven't already tested it on your receiver. At least it does this for Optical. (Though remember that the Optical test tone only plays 2 channels, so that's not a good test unless your receiver shows it's sending in 5.1 mode).