If you download all of the segments to Myk storage, then manually mix the first segment, Myk should automatically recognize the remaining segments (chapters) and combine to a single video. Sometimes the files can be too large for a phone to process, so you can always export the videos and single audio file to your PC for mixing.
I had a similar situation where a longer video (~10 min) split into two, but the audio is still a single file. I have both videos downloaded, and I'm trying to manually mix, but it is not automatically combining the two video files together. Is there something I am missing? Also, can you give some more detail onto how to export the videos and single files to my PC to mix?