This script takes a single input file and

1. Reduces the volume of the third audio-track in the file by half
2. Writes the reduced volume track into l
3. Mixes both the second audio-track and the lowered l-track together and stores them into a
4. Maps the original video to be the first track
5. Adds the newly mixed audio-track a as the sole audio track
6. Enables pass-through for the video-track
7. Configures the mixed audio-track to be AAC at 192kbit/s

Selector

[0:a:2] selects from the first file (0) the audio track (a) at 0-based index 2 (2)

Filter I/O

Filters are composed by input (left side), the filter-name, options and the output: [0:a:1] [l] amix=inputs=2 [a]

• Input 1: From the first input-file use the audio track and index 1
• Input 2: Use the previously created track l
• Filter Name: amix
• Options: inputs=2 sets input-count to two. (multiple options seperated by :)
• Output: a the new audio-track

Documentation

• Initial Idea (wrong filter!): https://blog.nytsoi.net/2017/12/31/ffmpeg-combining-audio-tracks
• amix-filter (correct one): https://ffmpeg.org/ffmpeg-filters.html#amix