Parts of this answer comes from: Sound doesn't work with Intel 5 Series/3400 chipset
Maybe you are experiencing an issue with your sound engine. Make sure you are using the appropriate sound profile for your case.
In this image we are choosing the appropriate sound device. Which will offer several sound profiles, based on the abilities of the sound device itself. Of those, we should at least choose one of those that mentions a sound input: "Analog Stereo Output + Analog Mono Input" is working for this case but you may have 4.0, 5.1 Sound profiles plus Analog Mono Input. Any of these that features a Mono Input will do the trick.
In this window we are going to choose the right place from where the computer is going to gather sound. Let's say: the place where the microphone is plugged in.
You can enter the sound preferences by clicking the Main Menu, then hit "System", "Preferences" and finally choose "Sound"
Give this a try and please inform if you face additional issues (like: having not "plus Analog Mono Input" sound profile).