Is it possible to record Window's output sounds programmatically in C#? A bit like recording something from the "what you hear" or "stereo output" feature (without having to select them)?
This is called loopback recording, and it is possible in Windows. If you have a soundcard that supports loopback (I just checked on my low-end Toshiba laptop, and it doesn't) you can record straight from the loopback device using the waveInOpen etc. API, which is easy to use in C#. Note: recording audio in this way necessarily entails a reduction in quality, since the audio signal is converted to analog for output and then re-digitized to support the loopback interface.
If you don't have a soundcard, WASAPI will let you do this. I suppose WASAPI can be used with C#, but it looks painful.