I have no sound card on my machine, but I want to (automatically) record screen with audio on it. So I thing it's some kind of virtual sound card driver needed to loop back sound send to play to microphone. Is there such solution for Windows OS?
Not sure I 100% understand your question, but you might want to check out:
sounds like this might fit your requirements.
A cheaper virtual sound card with the same function. http://www.shiningmorning.com/VirtualAudioStreaming/index.html
Also includes a virtual webcam to broadcast/record screen/file. May consider the virtual audio+webcam package.
possibly of interest:
Virtual Audio Card
It's free and it works on Windows XP and it creates 3 Virtual Audio Cables as Windows Sound drivers.
It comes from a software kit for a commercial TCP/IP mp3 streaming device. Go figure. Works for me in place of VAC (Virtual Audio Cable) but the latency on my machine is pretty awful. YMMV