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You can find this in the Device Toolbar, or in Device Preferences. If nothing is showing, you might need to enable it using the sound card's control panel as outlined below. Right-click inside the Recording tab and select Show disabled devices. Right-click again and check Show Disconnected Devices. If your sound card has a physical input such as mic or line in, connect the required cable as directed in the manual. Right-click the input device you want to use for recording and choose  Enable.   Right-click again over your chosen input device and select Set as Default Device. Right-click again over your input device, select Properties and then the Levels tab, and ensure the volume slider is turned up. Turn off any other sound effects as well, unless they are essential to the functionality of your sound card.  Right-click over Microphone and choose Properties then look for an Enhancements tab where you can Disable all sound effects.  On Windows 7, click the Communications tab. Under When Windows detects communications activity:, choose  Do nothing.  If you frequently make internet calls, right-click over the mic and choose  Set as Default Communication Device. Right-click your input device, select  Properties , then click the  Advanced  tab and verify that the  Default format  matches both the project rate (bottom left of the Audacity screen), and with the number of recording channels in the  Devices  tab of Audacity Preferences. Click  OK . In the Sound control panel, click the  Playback  tab, right-click over the speakers or headphones device for your sound card and set it as  Default Device  or  Default Communication Device. Right-click and click  Properties  then the  Advanced  tab, and set  Default Format  to match with the settings in Step 7 above.

Summary:
Select your input source. Show hidden devices. Plug in any necessary cables. Enable your input device. Turn off all VoIP enhancements. Adjust sample rates. Set your default device. Match formats.