Can't hear audio in Final Cut Pro X

When previewing some videos and some audio clips I cannot hear any audio - unless I first drag it to the timeline and then play it. Other video and audio clips play just fine in the preview area. I have no idea what would cause this - but maybe someone here might have a better idea.

EDIT: There's something funky with the Audio Configuration. Those videos that have the ability to playback sound while played in the preview window have a stereo option and a speaker icon with a number two. Those videos that cannot play sound (unless dragged onto the timeline) say "mono" and have a speaker icon with a number one. The dysfunctional clips can be heard in the preview window if you select Right or Left (there is no stereo option). But, then the audio is ONLY in the right or left speaker.

This is getting too complicated. I just wanted to preview the audio clip press the i and then the o key and drag an excerpt down to the time line - but not hearing the audio is a serious problem.