I think you need to go into Control Panel --> Sound, and set the SPDIF to be the default playback device. I've had similar problems years ago when I used to use SPDIF, and although I can't remember exactly what I did, it was to play around with the Sound applet in Control Panel. I think your answer lies there. I've had to do similar things when my laptop gets a little bit confused and stops sending sound out through the HDMI port to my receiver, usually because I've slept it and awakened it and it seems to have forgotten that I was using HDMI, so I get video out but sound comes out the laptop's speakers. I have to go into Sound, which sometimes has the HDMI device selected for playback, then I need to go into the Configure button and when I hit the Test button, sound comes out the HDMI cable where it was supposed to all along.
So play with the Sound applet, go into Configure or Properties if you have to, that should get things working.