You may want to download and install the driver from Dell. I think the window is using the generic driver to run the speaker so you get sound but the microphone is not working because there is non on the window.
I can ask Dell - but Dell Support may ask for make,model etc., of the mic - Mine is just one of those simpletons picked up from the market. May be there may be provision to say "Others" or "Not Known"! Shall try and get back to you!