I've been having this error recently and I thought it was due to Skype since when I had Skype open I couldn't hear sound. After the call I had to reboot to get sound.
Then I was just watching a youtube video when the sound suddenly stopped halfway through the video. My mixer bar still said that sound was being received from the video. anyway here are some things about the computer
Mx 17 Windows 7 alienware, I use the headset that came with the computer. It recently had all of its data erased due to a virus that was hidden where my security systems couldn't detect It.