How to disable microphone auto increase in volume?

What's causing it is that the option to allow programs control over the device is set as active. To disable this there are two steps. A windows step and a Skype step. You will also want to check any others (like teamspeak and the like) that also have auto adjust options.

On the Windows side:

• Goto Control panel
• Open Sound
• Click on Recording tab
• select your mic and click the properties button on the bottom.
• Uncheck the Allow applications to take exclusive control of this device

The Skype step:

• Click Tools at the top of Skype window
• Select Options
• On the left side of the Skype screen select Audio settings
• Under Microphone, uncheck Automaticly adjust Microphone settings

Control Panel > Hardware and Sound > Manage Audio Devices

On the communications tab try selecting "Do nothing".