This generally can happen from one of two reasons. One, you have multiple sound cards and your feed software isn't set to use the correct one. Two, your feed software is set correctly, but your audio settings for the sound card feeding it are incorrect, allowing your computer's microphone audio to be sent out over the feed (either instead of or in addition to your scanner audio).
Open the mixer and turn off the microphone input and turn the volume level for it down to the minimum level (just in case it becomes active again) and turn on the line-in and adjust the volume level to the correct levels. Make sure that the mixer is the one that controls the same sound card used for your feed. If everything looks correct there, make sure your feed has that sound card selected as its input device.