I have an MP4 recording where the video is present, but much too quiet. What I've done before is:
ffmpeg -i master.mp4 -i amplified.wav -vcodec copy output.mp4
I've tried various permutations, and the output ffmpeg builds is, as viewed by my browser, at the original overly quiet volume. If I load the Audacity-generated .wav, it plays as loudly as intended. (N.B. I have enabled clipping in Audacity, and I have not noticed it to fail at the amplifications I've tried with it.) However, the final result seems to retain unaltered the original audio track along in the video.
How, with present software or other software, can I assign amplified.wav to be the new audio track to master.mp4 in output.mp4?
The automatic stream selection was recently tweaked in June, and it inadvertently broke in some cases, especially for audio. Workaround is to explicitly map audio.
ffmpeg -i master.mp4 -i amplified.wav -map 0:v -map 1:a -vcodec copy output.mp4