Skype uses the sound devices on your computer in its own way. While you can adjust the levels of your speakers and microphone from Windows, those settings may not transfer into Skype, which has its own volume control dials. Additionally, Skype can use any of your sound devices, which may lead to disable other devices, like your mic, within Skype.
Check Levels in Skype
Click on "Tools" and then "Options" in Skype to load all of the program's settings. Click "Audio Settings" on the left-hand side and then speak a few words. Skype displays the microphone level along a green bar. If the levels remain low, uncheck "Automatically adjust microphone settings." If you don't see anything along the green level bar, click on the drop-down menu beside Microphone and choose another device. If your computer has two sound cards, or front and read audio jacks, Skype may use different drivers to control them. The same is true for computers with built-in webcams, which also have built-in microphones. If you use a headset on a computer with a built-in microphone, select the headset from the Audio Settings screen.