The problem : youtube video with an acceptable quality in your mother language will be not present on youtube, but the same film in a foreign language and with a better quality will be present, e.g.: Citizen X, german with additional Documentation, the film itself 1:36:00, bad video quality, french, 1:40:00

Both films cut with Avidemux at the "same" position (nearest position, depends on the respective IDR Frame), to have the same basic.

1. Got with ffmpeg audio only of the german film, video only of the french
2. Stretched with Audacity the audio-only stream, to have the same length as the french film, video only.
3. With ffmpeg merged the "french-film" and german-audio
4. Load the merged film with Avidemux and did correct the audio-offset.

Did check the film until the synchronization will be lost, go back to the last synch. part, and did cut the rest, saved as CitizenX1.
Load again the film with Avidemux to find the next audio offset for the second part of the film, a.s.o..

Is there a better way (tool, Ubuntu), to synchronize two different sources ?