There are several options. You can try to intall Linux like Ubunru 9.10 and broadcast from it.

You can try to install a commercial program called VirtualAudioCable.

And you can buy a sound card (internal or external). Example:
What USB sound cards from Creative support Stereo Mix/Wave-Out Mix/What U Hear?
Creative sound cards might not be the cheapest ones, but you can be sure that their sound quality will be good.