Just some stuff about me.
Adding an additional audio source on top of the audio already in a video can be accomplished with the amix filter. This might be useful for adding background music to a commentary video. The command is as follows:
$ ffmpeg -i input.mp4 -i input.mp3 -filter_complex "amix" -map 0:v -map 0:a -map 1:a output.mp4
A common issue that comes up with amix is the volume of one input over powers the second. Setting the louder input volume down can resolve this. In the example below the volume of the first input audio [0:a] is reduce 40%:
$ ffmpeg -i input.mp4 -i input.mp3 -filter_complex "[0:a]volume=0.40,amix" -map 0:v -map 0:a -map 1:a -shortest output.mp4
The option -shortest here changes the duration to the longest input media. For example, if the MP4 is longer than the MP3 the duration cuts to the MP3 length.
**Tip:** See question, *“What is -map and How is it Used?”*, for more details.