Is it the only audio device, and if not is it doing it in any sort of pattern?
Different programmes have their own settings for audio devices (handy, as I route normal PC sound through my speakers, but voice comms are set to a usb headset), and some programmes may disable the audio device if it's not there when the programme is started (IE your TV is off when you start a programme).
Remember there can be several different places for the sound to be set from depending on the version of windows, your installed sound device software, and the programme itself (very annoying with microphone settings at times).