Make sure you do not have a line output connected to the computer's Mic input. A line output must be connected to the Line input. USB audio devices with a line input
are available if you have a laptop that does not have a line input.
Make sure the input jack is not set to Microphone mode if you are not using a microphone. Use the "Sound" item under Windows Control Panel or any custom software incuded with the audio hardware to change that setting.