If you're using Adobe Premiere (which I assume you are), and the clips you're working with happen to have Dolby digital audio, then therein lies the problem: for whatever reason (some sort of a encoding issue or something), Dolby audio isn't supported in Premiere anymore unless you're using the latest version of whatever your machine's OS is. In my case, since I'm using Windows 7 and not 10, I have no audio whenever I edit footage I film with my camera, since it records audio in Dolby stereo - but this is also a problem when I download certain clips and videos online that also happen to have Dolby audio, like Dailymotion videos.