I have videos with 5.1-audio. That means: MPEG-4 AVC with AC-3. Of the 6 channels, I only need channel 1 + 2 (stereo). How should I proceed?
Multi-track audio is still a work in process, cf. Audio Routing | Channel Mapping (#754) · Issues · Multimedia / Kdenlive · GitLab
In the meantime, you might need to split the audio track off and downmix it separately yourself …