We had to install Ubuntu 15.04 on a brand new Lenovo B40-70 laptop. Everything worked fine until we tested sound on Skype. The speakers were working fine but apparently the internal microphone was not working. After banging the head on many doors we came across this wiki page entry about troubleshooting sound issues in skype that pointed out that pulseaudio was incorrectly detecting the mono mircophone as stereo microphone.
When we checked sound settings we saw that there were two channels and that they had identical amplification settings because both channels were locked together. The identical amplification on these two channels resulted in the audio input cancelling itself when processed as two separate channels. The fix was to unlock the channels and to reduce one of the channel amplification to 0% while keeping the other at 50%.
I hope this helps somebody else searching for a fix for the same symptom.