I want to pass sound from my System into my microphone so that people I am talking with can hear what I play. I don't want to use it like that, but I guess it is some kind of soundboard which sound is redirected into my microphone.
A microphone is an input device. Sound that is generated external to the microphone is then transmitted to the storage/recording device.
The electronics involved in this device make it a one-way device. You cannot pass sound into your microphone to be broadcast.
It's possible that you could be clearer in your requirements.
There is a program called Total Recorder which allows you to redirect sound that is being created on your computer (from video, from an application, from a microphone) to a destination of your choosing. If your objective is to stream audio that is generated from your computer, I suggest to clarify your question.
If the purpose is to transfer the audio from your microphone to speakers in the general area, it's possible that Total Recorder will do that, although audio feedback may result in an overpowering squeal!