Instead of worrying about DX10 supporting 7.1, you should worry about your users. Some hardcore gamers have 5.1, 6.1, or 7.1 sound, and to them it would be awesome if you employed such a soundscape in your aural pipeline, but to the general public who listens to audio through an onboard chip and basic stereo or 2.1 speakers, they wouldn't notice or wouldn't care. So I guess the real question is, who is your target audience for the game in question, but at this point I'm starting to ramble.
As it stands, the current FPSC uses the DBP "3D Sound" command set. This essentially loads the mono sound into the 3D space and plays that sound within said space, creating a certain level of submersion. On my audio setup (Creative SB Audigy LX and AA 5.1's), I'm getting proper 3D surround sound. If I'm standing in front and to the right of a sound, I hear that sound predominantly through the left-rear speaker. So I
think it has more to do with the user's soundcard and speaker configuration than DirectX, which I
think deals more with the management of the aural pipeline. But eh, I might be wrong about that, I'm not entirely sure.
"In an interstellar burst, I'm back to save the universe"
