Over the weekend I wanted to add a silly Noooo from Darth Vader in the prequels to a clip from Rogue One. I couldn't find any good tutorials on how to do it, so I figured it out for myself.

Adding Audio to a Video.sh

#!/usr/bin/env sh

# Extract audio from the Video File into a file.
ffmpeg -i video.mp4 -map 0 -vn -acodec -copy video_audio.m4a
# Create 11 seconds of silence
ffmpeg -f lavfi -i aevalsrc=0:0::duration=11 -ab 320k silence.mp3
# Prepend the silence to the audio track you downloaded
ffmpeg -i concat:"silence.mp3|commentary_audio.mp3" -codec copy silence_commentary_audio.mp3
# Merge the commentary track with the video_audio into a single file
ffmpeg -i silence_commentary_audio.mp3 -i video_audio.m4a -filter_complex amerge -ac 2 -c:a libfdk_aac -vbr 4 merged_audio.m4a
# REPLACE the original audio with the newly merged audio
ffmpeg -i video.mp4 -i merged_audio.m4a -map 0:v -map 1:a -c copy -shortest new_video.mp4