If you can post what components you are working with, computer model, soundcard info etc...
Is the microphone the critical component in this setup? It seems like you are trying to play audio, and either record in analogue a voice recording or instrumental?
Sometimes if you are recording an analogue sound source, it's better to play the audio through earphones and overlay directly onto the streamcast using a video editing program. Also placing a microphone in front of a speaker that it is transmitting through will cause some nasty feedback issue.