Glitch with sound while making slideshows with videoclips?

Is anyone else having this problem?

I make a slideshow with photos and videoclips. I add music to the slideshow. The music plays, and when the video clip comes on the music softens. However, no sound is played from the video clip. I know this clip works, if I double-click on it it plays fine in Quicktime.

However, with certain other video clips, the sound from those plays fine. I've seen from these posts that a few others seem to think that clips from one camera might play sound, but not another. That does not make sense to me.

Does anyone have any bright ideas in solving my problem? Thanks!