Reduced voice or dialogue in timeline

Sound, but no voice or dialogue when I put a video on a timeline. It's very strange. Everything is working great, but when I try to edit on a timeline - I barely can hear voices, only background sounds and music. It's like the program automatically doing that. But in some parts the voices are heard fine, but most of the time not. I usually working with movie editing. And it happens very often. I convert them from MKV to MP4 or AVI. I don't have that problems with animated cartoons though. All settings are default. Pls help. The program's player doesn't have that issue, It happens only if I try to edit video (put it on timeline)