Windows 7: Stereo mix is good, but gives me a secondary problem. - Any1 help?

Ok first I'll show what I know then I'll ask what I don't know:



Now I got it that for example if I want to record video games with sound as well I can just set the Stereo Mix as default recording device and just play and record (with a program like fraps).

But now I've got an other problem/curiosity.
I tried to play a joke on a friend with Skype by setting the Stereo Mix as recording device and play some funny voices with Media Player. Well all good until this point, only that a secondary effect gets out: he can hear his voice coming back to him.
Of-course ! All the stuff I hear at my speakers are heard by him also...


So! This is the question:

Is there a way to just infuse the sound into a microphone/recording device? or a program that does that?
Maybe a program that:
- when it plays a sound, the during it's playback that the only time-lapse when the Stereo mix is activated
- when it finishes/stops playing a sound the default recording device becomes again the microphone (or just nothing - because if it was Microphone he might hear me laugh). xD

I think you guys mainly got the idea about my problem.
Feel free to post any kind of way to get this result: "He won't be able to hear his voice come back to him"

^^ Hope somebody who is good in Windows 7 will be able to help me. Ciao!