If you're straining to hear someone over Skype, they could be a timid talker, or you might simply have your audio settings too low. There are two settings you should check before telling your friend to speak up. The Skype audio settings can be adjusted through the main interface or during a call. Windows settings could also be the culprit, so be sure to check your system volume as well.
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If you're in the middle of a call, click the "Call Quality Information" button, indicated by five progressively taller vertical bars on the toolbar. If you don't see the toolbar, move your mouse to the bottom portion of the screen to make it magically appear. Click the "Speaker" tab and drag the slider to the right to increase volume, or to the left to decrease volume. If you haven't started a call yet, click "Tools" from the main window and then select "Options." Click the "Audio Settings" tab, slide the "Volume" slider in the Speaker section and then click "Save."
Windows 8.1 Settings
In desktop mode, click the speaker icon in the notification area. If you don't see the speaker icon, click the small arrow to the left of the notification area to reveal all of the icons. Drag the slider icon up to increase the volume or down to decrease it. If this doesn't help, click "Mixer" under the slider, scroll until you see "Skype" and then adjust the Skype-specific slider. The Skype option will only be visible if Skype is open and currently producing sound.