Problem with external microphone

Hi,

 

I have Inspiron 1420, with built in webcam and digital Microphone.

 

I want to turn off the built in digital microphone, since I want to use IP Phone call and I want to use external microphone.

 

But even when I plug in external Mic, the built in mic works and the sound quality is not good.

 

Already when I inserted external mic, the sound went in olny from this mic and the quality was good.

 

Thank you.

I am looking forward to hear from you.