Internal microphone not working

This is what I had to do to get the internal mic to work on my Acer Aspire 5745G:

• Install pavucontrol >> sudo apt-get install pavucontrol
• Run sudo pavucontrol
• Go to the "Input Devices" tab
• Click the lock icon to unlock the channels
• Set the right-front channel to silence

Add this line to your /etc/modprobe.d/alsa-base.conf file:
options snd-hda-intel model=dell-vostro enable=1 index=0

I had the same problem, and fixed it thus:
If you go to the Ubuntu Software Centre and download the Pulse Audio Volume Control (or from a terminal type sudo apt-get install pavucontrol). Then just run it, and your microphone should show up there.

If your microphone shows up but you're still not getting any sound, unlock the channels, and drop one of them down to zero. This worked for me, hope it helps you!