Audio out of sync when using ffmpeg, adelay, and amix

Question

I have a series of mp3 files with a start time offset.

I am trying to merge them together with adelay and amix.

The following codes run and creates the combined MP3 but when I listen to the final output file there are spots where people are talking over each other not in the sense they are being rude but the audio being out of sync.

Here are the files with the start and stoptime offsets:

d4a19a58-790a-45c1-b96e-819bc98cd408.webm | 1720 | 3762173
ab794f92-bdfb-4bb0-821d-ba80c8a9228a.webm | 48920 | 3328385
a7df9a51-0671-4930-a14b-e7d85a562577.webm | 76220 | 3763361
5d312878-d25d-40d7-9418-bed85e22efc6.webm | 3109640 | 3267473
1cf6127d-250a-4601-a219-b1b6bbd725f8.webm | 3360980 | 3636927
c3ddd462-e05a-425f-af58-f288f930f2cb.webm | 3689600 | 3760423

Here is the command I used to get the webm file to convert to MP3 broken into lines for readability:

ffmpeg 
-acodec libopus 
-i d4a19a58-790a-45c1-b96e-819bc98cd408.webm 
-b:a 320K 
-vn 
-y d4a19a58-790a-45c1-b96e-819bc98cd408.mp3

Here is the command broke to combine all the mp3s into lines for easier readability:

ffmpeg 
-i d4a19a58-790a-45c1-b96e-819bc98cd408.mp3 
-i ab794f92-bdfb-4bb0-821d-ba80c8a9228a.mp3 
-i a7df9a51-0671-4930-a14b-e7d85a562577.mp3 
-i 1cf6127d-250a-4601-a219-b1b6bbd725f8.mp3 
-i c3ddd462-e05a-425f-af58-f288f930f2cb.mp3 
-filter_complex "
[0]adelay=1720|1720[a0];
[1]adelay=48920|48920[a1];
[2]adelay=76220|76220[a2];
[3]adelay=3360980|3360980[a3];
[4]adelay=3689600|3689600[a4];
[a0][a1][a2][a3][a4]amix=inputs=5
" 
out.mp3 -y

Any ideas on what I am doing wrong?

Answer 1

you need to delay all the audio channels with the same value using adelay=milliseconds:all=true, and use -async 1 at the end of your command so ffmpeg will just corrects the start of the audio stream instead of stretching/squeezing.

so in your case :

ffmpeg 
-i input1.webm 
-i input2.webm
-i input3.webm
-i input4.webm
-i input5.webm
-filter_complex "
[0]adelay=1720:all=true[a0];
[1]adelay=48920:all=true[a1];
[2]adelay=76220:all=true[a2];
[3]adelay=3360980:all=true[a3];
[4]adelay=3689600:all=true[a4];
[a0][a1][a2][a3][a4]amix=inputs=5 [out]
"
-map "[out]"
out.webm -async 1 -y

as the documentations says :

-async samples_per_second

Audio sync method. "Stretches/squeezes" the audio stream to match the timestamps, the parameter is the maximum samples per second by which the audio is changed. -async 1 is a special case where only the start of the audio stream is corrected without any later correction.