Microphone does not work at Dell XPS X8700

Hi there, couple of months ago I bought XPS-X8700 from Amazon, see link below if you need specification, which I have been using to deliver math tutoring via Skype or Google Hangouts.

www.amazon.com/.../B00K0HMVNM;redirect=true&ref_=oh_aui_detailpage_o04_s00

Everything works fine except it looks like that there is microphone issue like these:

(1) When I connect it to a BlueTooth headset, the speaker works fine but the microphone does not work at all.

www.amazon.com/.../B00TUGU84I;redirect=true&ref_=oh_aui_detailpage_o08_s00

(2) Then I switched to plain simple wire headset by Plantronics, I had the same problem, ie., the speaker works but the mic does not.

(3) Finally I have to switch back to a very old Logitech headset and it works on both speaker and microphone.

Any help or fix would be very much appreciated. Thank you for your time and effort.