The steps are followed in Windows 7 computers as listed below. It can be similar in the other versions of Windows.
Right-click on the speaker icon at the right end of the Windows task bar.
Select the recorders from the menu appeared. Go to the Record tab from the window appeared.
Control this section to if there is a row named Stereo Mix.
If there is, right-click on the row and select the Enable.
If there is not, update the driver of your sound card.
Then check the same section. If the Stereo Mix row appeared, right-click on it and select Enable.
After you make this adjustment, connect to the computer on the other side with alpemix and if the option start the voice communication on the alpemix window on the right bottom of the connected computer is selected you can start hearing the voices on the other side.