Help! I have a Jensen turntable attached to my laptop via a usb port. The turntable isn’t on the list of input devices on the drop-down input sources on audacity. But the sound it generates is recorded. My problem is preventing the built-in microphone from adding to extraneous sounds on the recording. I tried muting the mic but that kiled all the sound inputs. Thanks for your help. 
Use the device toolbar to select the input that you want to use.
Note that Audacity only automatically scans for available devices when Audacity is launched, so normally you would ensure that the turntable is connected (and recognised by Windows) before you start Audacity. A rescan of devices may be manually performed by selecting “Rescan Audio Devices” near the bottom of the “Transport” menu.