If you are recording from a USB device these can change the system playback device away from the inbuilt sound device to themselves, even if they are not intended as playback devices for the computer. Please recheck that
* the playback device in Sounds and Audio devices is set to Realtek and not muted
* the playback device in Audacity Preferences > Playback Device is Realtek
* the output volume slider in the Audacity Mixer Toolbar is turned up
* the playback device in Windows Media Player Tools > Options is Realtek
* the files have a waveform with peaks and troughs in Audacity - if they
are a flat line you did not record properly
* you did not turn the gain down on the Audacity Track Panel when
exporting, or exported the wrong track
Gale