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