Let me guess…
Are the tapes from a Canon camcorder?
But more…
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.
Many people have reported that noise or “pops” appear in the captured clips if the Mac’s internal audio is left on WHILE capturing… so may want to choose “Off During Capture.”
Here is the way you can turn OFF the Mac’s speaker:
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 “MacAudioOFF” to the file name).
Click open the “Advanced” button in the Audio section and turn speakers to “OFF during Capture.”
The Mac speakers/audio-out will now NOT play Audio during the preview in the Log and Capture window. (Monitor from the camcorder or deck.)