If i remember rightly there is a application that i downloaded once which allowed me to play my speaker output to my microphone input, so in other words when i was gaming i could play what music i was listening to on my pc clearer to my fellow gamers, i cannot for the life of me remember what it was called, i will get the link if i can dig it, if it did that im sure it may have some more potential?
In the meantime it may be worth experimenting with the audio mixer in windows? So at least then if your primary focus is movie you can make that louder?
edit: forgive me had a "der" moment, depending on the game you are playing, you should be able to configure what output it goes to, at least in my LOTRO days i could configure what speakers i would like the audio output to go to.
My thinking would be if you set the one you want your movie on to your "default" audio device, then in the online game tell it to only output through the other, then maybe it will work?