I want to share a problem that I stumbled on recently. I was helping my mother setting up her computer to allow for recording audio through stereo mix. Stereo Mix is a virtual recording device that allows you to record all sound currently being played on the computer, sort of what-you-hear-is-what-you-get. I got it working and was able to record the sound without problems. Fast forward to the next day, and I continue working with the computer. But this time the sound I recorded was just silent! Even though stereo mix was selected as input device and all other settings were the same, it no longer recorded anything.
The problem turned out to be this: When I set up the stereo mix I had used my own headphones. These are normal headphones with a classic stereo plug. The second time though, I plugged in my mother’s headset instead. This is a headset that is connected to the computer via the USB-port. And this was the problem. As soon as they were connected, no sound was recorded. Unplugging them solved the problem!
A thread on Superuser points towards an explanation: Stereo mix can only record audio that travels through the internal sound card. I suspect the USB headphones act as sound card, and thus the sound travels directly to them instead. So even though the stereo mix actually works, there is no sound flowing through the device to record.