Its actually a lot more difficult then you think. Winamp just has to recognise one word (eg, "close" or "play"), while in DefCon you would have to split up sentences ("Fire nuke at Moscow", etc) to work out exactly what the play wants to do. And then you would have to work out where everything is on the map.
And how would you move a sub? "Submarine. Move to 50km North of X"? Thats probably tripling the complexity of DefCon just with giving units path finding, I would expect.
And obviously, each player would have to train up DefCon or the Windows speech recognition software first. Which takes HOURS. I did only the basic training in Windows to see if I could just "speak out" my essays, and it took ages, and still made loads of mistakes...
Thats not even getting to the fact that Iv have a history of releasing games for PC, Mac and Linux, which would mean getting some form of speech recognition software that runs in Linux and some that runs on Macs, and then making sure they all work! Which is again, tripling the amount of work.
Some RTS (I forget which) tried using speech recognition, and it just annoyed most people

Which is a shame...
Still, nice idea, but its probably more work then its worth

We dont stop playing cos we get old... We get old cos we stop playing.