Hi Mike, can you tell me please what OS you are using? I was trying the same yesterday, but in my case it's the Dodo engines coming through the headphones. Most planes work fine, but the Dodo is the first I tried that didn't. I put it down to the developers using "voice" channels for certain sounds, and thought no more about it.
Now you say you did something with the speaker icon? I was wondering if that was a Windows 7 thing, or if it would work in XP. I am trying to get voice only through the USB headphones, and all other sounds through my 7.1 surround system.
Thanks for any insight.