NUC5i5RYH mic. and earphones.

I have just set up a NUC5i5RYH and wish to use a microphone and headphones with Skype.  I am using a CTIA standard splitter cable to allow connection of both devices.  I get notification that the devices have been plugged in.  However, I get no input from the microphone.  This situation appears to occur with any microphone and even if the microphone is used without the headphones.  The primary microphone I wish to use is a desktop stereo model - important for round table settings.  I have been assured by the microphone manufacturers that there is no problem plugging the mic into a mono socket.

Any suggestions as to what is happening and how to fix it would be appreciated.

Thank you

Peter