can't hear FaceTime on Macbook Pro Retina Mid-2015
The audio options are available in Facetime in a perhaps confusing place:

Open Facetime. Then in the top menu choose "Video" and at the end of this menu, there's a subtitle "Output" where you can choose the audio device. Choose the speakers you want here, and you should be able to hear people on calls.

If you still cannot hear the other party, then open System Preferences > Sound > Output, and select the device you've selected in Facetime also. Now you should see an "Output volume" slider, where you can adjust the volume.