We had a call about this a few days ago. It appears that some of the voice overs are recorded in "mono" instead of "stereo". So if your computer only has one speaker or your sound is set to mono you may not be able to hear.
Check your sound properties by clicking on the speaker icon located at the bottom right of your computer.
You must double click the icon to get to the advanced options tab and check all your settings in there to make sure everything is on and balanced especially the "WAVE" settings. They should be moved in the center to allow both sound channels to come through.
I hope this works. We'll look into this issue further when we edit the next video.
