I had this problem for a few weeks, i recently read a forum post telling me to untick the :allow applications to take exclusive control of device:
Well, I found that setting and changed a couple of others, and I think it's fixed it

I went via Control Panel > Hardware and Sound > Manage Audio Devices to get to the Sound panel.
I then selected Playback > Speakers > Enhancements and made sure everything was un-checked.
I then selected Playback > Speakers > Advanced and un-checked 'Allow applications to take exclusive control of this device'
I went to Recording > Microphone > Listen and ensured 'Listen to this device' was unchecked.
I went to Levels and ensured the Microphone level was at 100% and the Boost was at +10db
I went into Enhancements and ensured everything was un-checked.
I then into Advanced and un-checked 'Allow applications to take exclusive control of this device'
And finally I went into the Communications tab of the Sound panel, and changed 'When windows detects communications activity' to 'Do nothing'