apt-get install alsa-tools-gui
Open HDAJackRetask from the Main Menu (in Mint Cinnamon it's under Sound & Video). Select your sound card and then, under the Options panel, check the Show unconnected pins option. In the Pin configuration section, look for your Internal Mic, Riser entry, click the Override checkbox, and set the spin button to Not connected.
Next, look for the Pin ID: 0x1d, check the Override checkbox, and the use the spin button to select Internal mic. Now click the Apply now option and test your microphone. If all's gone well, it's time to make this a permanent change: click the Install boot override. You'll see the following message dialog: click OK and reboot.
http://karmicodyssey.blogspot.pt/2014/10/fixing-internal-microphone-pre…