imovie

I imported video clips with Mpeg-2 audio but at the timeline the audio doesn't play smoothly. Sometimes there's silence. Very often I'd see this green-blue-yellow-red spinning ball before the clips play. Sometimes it plays OK. I don't have this problem with the audio in video clips taken by my iPhone and another source. Why's that?

Very important, I'd like to know if I export or share the video, will the audio be OK?

Cheers!

James