The audio is, by default, MUTED to the Mac’s speaker output during Log and Capture.
You should normally monitor the audio directly from the camcorder or deck you are using to capture.
NOTE:
Many people have reported that noise or “pops” appear in the captured clips if the Mac audio is left on WHILE capturing… so may want to choose “Off During Capture.”
However, there is a way you can turn ON the Mac’s speaker if you really WANT to:
Final Cut Pro (menu) > Audio/Video Settings… > Capture Presets (tab)
“Duplicate” the Capture Preset you are now using, when the window opens rename it (I added “MacAudioOn” to the file name).
Click open the “Advanced” button in the Audio section and turn speakers to “ON during Capture.”
The Mac speakers/audio-out will now play Audio during the preview in the Log and Capture window.