iMovie exports fine in all qualities except for HD, which has no sound at all

Okay so I go into my project, which is a video for YouTube, and I can edit everything fine and it is great (Except for adding pictures, which almost everyone is saying they can't figure that one out). The sound in iMovie has no issue, but every time I export my video in HD, which I used a Nikon D7100 to record with so it should be perfectly fine in HD, the video file always comes out with no sound. If I export the video in the large file, it is perfectly fine and the sound is there, but I have to pay the loss of quality in order to get the sound...?

I have not been able to find anyone with this same issue and I have been researching for about 2 hours now (That includes iMovie making all my inserted photos black, if you can help with that also.)

Please help if you can!!

Which versions of OS X and iMovie are you using?