I have a problem: I add the music files and videos to the list, but when I add them to timeline (video and audio path) I can not watch them smoothly and listen when reviewing the project. That’s why I can not fit the image to the music. Video comes from gopro 120 frame/sek. What I am doing wrong? I can watch and listen to video and music without any problems if i choose them from the list not time line.
You probably have Settings > Video mode set to “Automatic” That means it is trying to make a project at 120 fps since that is your clip’s frame rate, which is probably more than you need. You should probably choose a different video mode at like 29.97 or 30 fps. If you do and you reduce the speed of a clip, then it will be nice and smooth. 
You right. Audio sounds better at 30fps but it is still not smooth. I must find what combination of fps and movie speed use to make it work.
Are you using more than one video track?
I am using 1 video track and 1 audio but multiple video and songs files. Videos are all from gopro 720 120frames/sek.
OK, then there is nothing else you can do, and that is the way Shotcut is.