Have you checked to make sure your HDMI/Denon AVR is still the default audio playback device?
I'm not sure which version of Windows you're using, but in Windows 7 (while your AVR is on/connected) right click on the speaker icon in your taskbar, pick 'playback devices' and you should get a window with the various outputs from your computer ('speakers', 'headphones' and hopefully your HDMI out). Right click on the HDMI/AVR output and select 'set as default device' - once you do this a little green circle with a checkmark in it should appear next to the device. It should (obviously there are no guarantees in Windows, as in life) from then on switch your output to the HDMI/AVR whenever the device is connected.
I wish that were the problem. But, yeah. The AVR is the default, and that never changes.