The point being that wd media player is is Hdmi Only audio as well as optical Only. Hdmi video is constant no mater what audio you select whether it is hdmi, optical or stereo so what ever way, you will need to go into your wd media player settings to change the audio.The easiest way is to connect your hdmi to your avr and change the settings accordingly.
That being said since you are ordering cables it wouldn't hurt to have an optical around since they are fairly cheap and you will never know when you may need one.
Actually, with optical I get 6.1 stereo, with HDM I think it was 4.0 or 5.0, no subwoofer. BTW, I was doing this test for the OP Stealth3si, not for me. I am done with it for now.