I have been using for over a year now and very happy with the app! Work great for making these special piano videos.
Since some time I have a new computer and download 4.9.6 of Seemusic. While rendering it doesn't include the background video I added. It shows everything else, except for the background mp4. I rendered as H.264 video codec. Later I tried as HEVC and downloaded the HEVC extension from Windows. Then it rendered once with the background, but now for another version of my video it leaves out the background again.
Any tips on how to fix this issue? While editing the video it shows the background perfectly. Just while rendering and in the rendered video there is no background. But everything else is there (hands, particles, midi).