How Can I Output Sound to My Home Theater Onkyo AV Receiver Via HDMI While Still Using My Laptop Screen for Viewing?

Dell 15r N5110. W7 64 HP.

If I want to listen to internet radio on my HT system via HDMI - while working on my laptop - I cannot effectively use my laptop screen. I cannot click between tabs, nor move within them in Waterfox and IE. (The mouse GUI seems disconnected over the browsers, yet works fine elsewhere). If I use the HDMI and tv screen only,  picture and tabbed browsing is fine, but I would like to work off my laptop screen and prefer not to have the tv on. If want to use the laptop as the primary screen, I cannot figure out how to output the sound via the HDMI cable, as the TX-SR805-1, NVIDIA Hi Def Audio, says Not plugged in  -  even though it is plugged in!