Koldflux add-audio-to-video test log, run 2026-10-21. ffmpeg version 6.1.1-3ubuntu5 Copyright (c) 2000-2023 the FFmpeg developers
All inputs are generated here by ffmpeg lavfi; no third-party media. Picture check = md5 of the decoded video stream (ffmpeg -f md5).
Shipped file names: silent.mp4 is silent-video.mp4, music8.mp3 is music-8s.mp3, D.mp4 is result-looped.mp4.
## inputs
silent.mp4
codec_name=h264
codec_type=video
duration=12.000000
size=20922
music8.mp3
codec_name=mp3
codec_type=audio
duration=8.045714
size=129193
music20.mp3
codec_name=mp3
codec_type=audio
duration=20.035918
size=321036
withsound.mp4
codec_name=h264
codec_type=video
codec_name=aac
codec_type=audio
duration=12.000000
size=218945
## A: music20 (longer than the 12 s video) onto silent video, picture copied, -shortest
$ ffmpeg -i silent.mp4 -i music20.mp3 -map 0:v -map 1:a -c:v copy -c:a aac -b:a 192k -shortest A.mp4
exit=0
## B: music8 (shorter than the video), no -shortest
$ ffmpeg -i silent.mp4 -i music8.mp3 -map 0:v -map 1:a -c:v copy -c:a aac -b:a 192k B.mp4
exit=0
## C: music8 with -shortest (the trap)
$ ffmpeg -i silent.mp4 -i music8.mp3 -map 0:v -map 1:a -c:v copy -c:a aac -b:a 192k -shortest C.mp4
exit=0
## D: loop music8 to fill the video
$ ffmpeg -i silent.mp4 -stream_loop -1 -i music8.mp3 -map 0:v -map 1:a -c:v copy -c:a aac -b:a 192k -shortest D.mp4
exit=0
## E: mix music20 at 30% with the original sound
$ ffmpeg -i withsound.mp4 -i music20.mp3 -filter_complex [0:a]volume=1.0[a0];[1:a]volume=0.3[a1];[a0][a1]amix=inputs=2:duration=first:normalize=0[a] -map 0:v -map [a] -c:v copy -c:a aac -b:a 192k E.mp4
exit=0
## F: start music8 3 s into the video, 2 s fade out ending at 12 s
$ ffmpeg -i silent.mp4 -i music8.mp3 -filter_complex [1:a]adelay=3000:all=1,apad=whole_dur=12,afade=t=out:st=10:d=2[a] -map 0:v -map [a] -c:v copy -c:a aac -b:a 192k -shortest F.mp4
exit=0
## G: copy AAC audio without re-encoding
$ ffmpeg -i silent.mp4 -i music8.m4a -map 0:v -map 1:a -c:v copy -c:a copy G.mp4
exit=0
## H: copy WAV straight into MP4
$ ffmpeg -i silent.mp4 -i music8.wav -map 0:v -map 1:a -c:v copy -c:a copy H.mp4
exit=0
## I: video that already has sound, no -map (default stream choice) vs explicit -map
$ ffmpeg -i withsound.mp4 -i music8.mp3 -c:v copy -c:a aac I_default.mp4
exit=0
$ ffmpeg -i withsound.mp4 -i music8.mp3 -map 0:v -map 1:a -c:v copy -c:a aac I_explicit.mp4
exit=0
## results
picture md5 of silent.mp4: MD5=3fb21bc9fda2c61fa80b0fb8991799ee
A: format(duration,size)=12.000000,237632  video_dur=12.000000 audio(codec,dur)=aac,11.980998  audio_streams=1 picture_vs_silent=identical mean_first2s=-11.4 dB mean_from_10s=-11.4 dB
B: format(duration,size)=12.000000,165395  video_dur=12.000000 audio(codec,dur)=aac,8.000000  audio_streams=1 picture_vs_silent=identical mean_first2s=-11.4 dB mean_from_10s=
C: format(duration,size)=8.000000,158887  video_dur=7.920000 audio(codec,dur)=aac,8.000000  audio_streams=1 picture_vs_silent=different mean_first2s=-11.4 dB mean_from_10s=
D: format(duration,size)=12.000000,236853  video_dur=12.000000 audio(codec,dur)=aac,11.982993  audio_streams=1 picture_vs_silent=identical mean_first2s=-11.4 dB mean_from_10s=-11.4 dB
E: format(duration,size)=12.000000,239750  video_dur=12.000000 audio(codec,dur)=aac,11.911995  audio_streams=1 picture_vs_silent=identical mean_first2s=-18.4 dB mean_from_10s=-18.5 dB
F: format(duration,size)=12.000000,163290  video_dur=12.000000 audio(codec,dur)=aac,12.000000  audio_streams=1 picture_vs_silent=identical mean_first2s=-91.0 dB mean_from_10s=-16.8 dB
G: format(duration,size)=12.000000,156779  video_dur=12.000000 audio(codec,dur)=aac,8.000000  audio_streams=1 picture_vs_silent=identical mean_first2s=-11.0 dB mean_from_10s=
H: format(duration,size)=12.000000,729053  video_dur=12.000000 audio(codec,dur)=pcm_s16le,8.000000  audio_streams=1 picture_vs_silent=identical mean_first2s=-11.0 dB mean_from_10s=
I_default: format(duration,size)=12.003991,132009  video_dur=12.000000 audio(codec,dur)=aac,12.003991  audio_streams=1 picture_vs_silent=identical mean_first2s=-21.0 dB mean_from_10s=-21.1 dB
I_explicit: format(duration,size)=12.000000,95822  video_dur=12.000000 audio(codec,dur)=aac,8.000000  audio_streams=1 picture_vs_silent=identical mean_first2s=-11.4 dB mean_from_10s=
-rw-rw-r-- 1 ubuntu ubuntu  237632 Oct 21 12:40 A.mp4
-rw-rw-r-- 1 ubuntu ubuntu  165395 Oct 21 12:40 B.mp4
-rw-rw-r-- 1 ubuntu ubuntu  158887 Oct 21 12:40 C.mp4
-rw-rw-r-- 1 ubuntu ubuntu  236853 Oct 21 12:40 D.mp4
-rw-rw-r-- 1 ubuntu ubuntu  239750 Oct 21 12:40 E.mp4
-rw-rw-r-- 1 ubuntu ubuntu  163290 Oct 21 12:40 F.mp4
-rw-rw-r-- 1 ubuntu ubuntu  156779 Oct 21 12:40 G.mp4
-rw-rw-r-- 1 ubuntu ubuntu  729053 Oct 21 12:40 H.mp4
-rw-rw-r-- 1 ubuntu ubuntu  132009 Oct 21 12:40 I_default.mp4
-rw-rw-r-- 1 ubuntu ubuntu   95822 Oct 21 12:40 I_explicit.mp4
-rw-rw-r-- 1 ubuntu ubuntu  321036 Oct 21 12:40 music20.mp3
-rw-rw-r-- 1 ubuntu ubuntu 1764078 Oct 21 12:40 music20.wav
-rw-rw-r-- 1 ubuntu ubuntu  133135 Oct 21 12:40 music8.m4a
-rw-rw-r-- 1 ubuntu ubuntu  129193 Oct 21 12:40 music8.mp3
-rw-rw-r-- 1 ubuntu ubuntu  705678 Oct 21 12:40 music8.wav
-rw-rw-r-- 1 ubuntu ubuntu   20922 Oct 21 12:40 silent.mp4
-rw-rw-r-- 1 ubuntu ubuntu  218945 Oct 21 12:40 withsound.mp4

## Koldflux browser tool, run on a local development build (headless Chromium via Playwright, page security policy bypassed for the dev build only) at /tools/video/add-audio on 2026-10-21
Inputs: silent.mp4 (12 s), withsound.mp4 (12 s, 180 Hz tone), music8.mp3 (8 s), music20.mp3 (20 s). The time shown includes page load and the first engine download, not only the conversion.
## T1 replace, music 8 s, repeat ON, video silent 12 s: clicked Add the audio to Download link visible in 14.9 s (includes page load and first engine download)
codec_name=h264|codec_type=video|duration=12.000000
codec_name=aac|codec_type=audio|duration=12.006009
duration=12.006009|size=237285
mean level first 2 s: -11.4 dB; from 10 s on: -11.4 dB
picture md5: MD5=3fb21bc9fda2c61fa80b0fb8991799ee (source MD5=3fb21bc9fda2c61fa80b0fb8991799ee)
## T2 replace, music 8 s, repeat OFF, video silent 12 s: clicked Add the audio to Download link visible in 13.0 s (includes page load and first engine download)
codec_name=h264|codec_type=video|duration=12.000000
codec_name=aac|codec_type=audio|duration=12.003991
duration=12.003991|size=167974
mean level first 2 s: -11.4 dB; from 10 s on: -91.0 dB
picture md5: MD5=3fb21bc9fda2c61fa80b0fb8991799ee (source MD5=3fb21bc9fda2c61fa80b0fb8991799ee)
## T3 mix, music 20 s at 100% with original 12 s sound (original 100%): clicked Add the audio to Download link visible in 12.5 s (includes page load and first engine download)
codec_name=h264|codec_type=video|duration=12.000000
codec_name=aac|codec_type=audio|duration=11.958005
duration=12.000000|size=239217
mean level first 2 s: -11.0 dB; from 10 s on: -11.0 dB
picture md5: MD5=3fb21bc9fda2c61fa80b0fb8991799ee (source MD5=3fb21bc9fda2c61fa80b0fb8991799ee)

## Codec and container check with -c:v copy, run 2026-10-21, ffmpeg version 6.1.1-3ubuntu5 Copyright (c) 2000-2023 the FFmpeg developers
Sources are made from silent.mp4's generated gradients. Each command is followed by its exit code, first error line and ffprobe of the result.
src_prores.mov codec: prores
src_hevc.mp4 codec: hevc
src silent.mp4 codec: h264
$ ffmpeg -i src_prores.mov -i music8.mp3 -map 0:v -map 1:a -c:v copy -c:a aac -shortest
exit=234; first error: [mp4 @ 0x63fc6958cc80] Could not find tag for codec prores in stream #0, codec not currently supported in container
result: no usable file
$ ffmpeg -i src_hevc.mp4 -i music8.mp3 -map 0:v -map 1:a -c:v copy -c:a libopus -shortest
exit=234; first error: [webm @ 0x5b7a6e7f33c0] Only VP8 or VP9 or AV1 video and Vorbis or Opus audio and WebVTT subtitles are supported for WebM.
result: 
$ ffmpeg -i silent.mp4 -i music8.mp3 -map 0:v -map 1:a -c:v copy -c:a libopus -shortest
exit=234; first error: [webm @ 0x5b37bbc31e80] Only VP8 or VP9 or AV1 video and Vorbis or Opus audio and WebVTT subtitles are supported for WebM.
result: 
$ ffmpeg -i src_hevc.mp4 -i music8.mp3 -map 0:v -map 1:a -c:v copy -c:a aac -shortest
exit=0; first error: 
result: codec_name=hevc codec_name=aac duration=8.000000|size=92196 
## fallbacks (re-encode the picture)
$ ffmpeg -i src_prores.mov -i music8.mp3 -map 0:v -map 1:a -c:v libx264 -crf 23 -pix_fmt yuv420p -c:a aac -shortest
exit=0; first error: 
result: codec_name=h264 codec_name=aac duration=8.000000|size=103061 
$ ffmpeg -i silent.mp4 -i music8.mp3 -map 0:v -map 1:a -c:v libvpx-vp9 -crf 35 -b:v 0 -c:a libopus -shortest
exit=0; first error: 
result: codec_name=vp9 codec_name=opus duration=8.008000|size=98627 
(VPS clock date in the header above is UTC; run done the same day as the other tests.)