SOLVED -- at least for me. I received a PM from the Dell social media team with the following advice... "try installing the Waves MaxxAudio Pro Application (if not already installed), then ensure this and the RealTek audio is enabled in Task Manager > Startup tab as well as in the Windows services.  Once done, try connecting the headphones to the port and check if you get a prompt from Waves MaxxAudio Pro to select which device you've inserted. Ensure to select the appropriate option and then test to see if the microphone works." This worked for my XPS 8940 -- much appreciated! -Dan
I have a Razor Kraken X headset and when i plug it in i have sound but my microphone is not working. The headset works correctly, including microphone on my work computer.  
XPS 8940, Headphones work, microphone does not