Thanks. That did it! The sound card (such that it is) is byRealtek (on the mother board -not a separate sound card). In the recording section of the dialog for the Realtek mixer, there are sliders for 'Line In", "Mic" and "Stereo Mix" . There is a "mute" button for each input source. Clicking mute for the Stereo Mix solved it.

Thanks again for your help. Andy