Problem with laptop combo jack

[Total War UK]

I've recently bought a Sony laptop but the internal microphone is of terrible quality. I've got an external microphone however the laptop onky has a headphone/microphone combo jack. when I plug the external microphone in my built in speakers stop producing sound however in the sound settings they continue to show output. In the sound settings in the control panel the microphone doesn't show in the recording tab.
How Can I use my external microphone while still using my built in speaker.

 

[jmourao]

Do you have your laptop latest sound drivers?