Right click the volume control in the toolbar -> Recording devices.
Make sure you have "Show disabled devices" checked/ticked.
Do you see any of the following devices?:
- Stereo mix.
- Wave out mix.
- Mono out.
- What U Hear.
If so, use that as your "microphone" and it will basically input any sound which is being outputted through your computer.
If you don't have any of those devices, then try installing
Virtual Audio Cable and follow a guide on how to set it up. It is a nice tool and works well with SAM Broadcaster if set up correctly.
Hope this helps
