1. Install pulseaudio volume control
sudo apt-get install pavucontrol2. Open it
Applications -> Sound & Video -> PulseAudio volume control3. Goto tab 'Input Devices' and select 'Hardware Input Devices' from the Show control at the bottom.
4. Unlock the stereo channels by clicking on on lock icon
5. Adjust one channel to zero and the other to 90% to 100%
Skype
1. Install Skype
2. Go to Skype's Options -> Sound Devices and uncheck 'Allow Skype to automatically adjust mixer levels'
3. Hit apply and make a test call