需要帮助使用 ffmpeg 来“连接”多个音频文件（webm、mp4）以制作更长的 MP3 音频

Question

I have done some searching (for several hours) and tried to manipulate many examples to work for me, but I still keep coming up empty here.

I am using linux-mint 19, with ffmpeg installed. I have a folder with several audio files. The majority of these are "webm" (with no video) and there are a few "m4a". I am trying to make one long mp3 file from the audio in all of these strung together from start to finish.

Lets say for sake of argument, my directory has the following files:

audio file a.webm
audio file b.webm
audio file c.m4a
audio file d.webm

I found a script that I could write online where someone is creating a file called "mylist.txt" with this bit of code:

    # with a bash for loop
for f in ./*.*; do echo "file '$f'" >> mylist.txt; done
# or with printf
printf "file '%s'\n" ./*.* > mylist.txt

this generated a text file with the following type of content:

file './audio file a.webm'
file './audio file b.webm'
file './audio file c.m4a'
file './audio file d.webm'

first, I believe the "./" is causing a problem, because when I look at other examples I don't see this but I am not sure why it is generating this way in my script because I don't see this in web examples online. Second, I have tried to "concate" this with ffmpeg but I'm not sure which is the best option. I found some documentation here:

https://trac.ffmpeg.org/wiki/Concatenate

however this example applies to video specifically

Can anyone lead me in the right direction?

EDIT******

I tried the solution below, with "mylist.txt" as the input and I am getting an error:

    user@machine/TEMP$ ffmpeg -i mylist.txt -filter_complex "[0:a][1:a][2:a][3:a]concat=n=20:a=1:v=0[a]" -map "[a]" output.mp3
ffmpeg version 4.2.4-1ubuntu0.1 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-10ubuntu2)
  configuration: --prefix=/usr --extra-version=1ubuntu0.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, tty, from 'mylist.txt':
  Duration: 00:00:00.40, bitrate: 47 kb/s
    Stream #0:0: Video: ansi, pal8, 640x400, 25 fps, 25 tbr, 25 tbn, 25 tbc
Stream specifier ':a' in filtergraph description [0:a][1:a][2:a][3:a]concat=n=20:a=1:v=0[a] matches no streams.

Answer 1

The concat demuxer works best with inputs that all have the same attributes. The concat demuxer documentation states, "All files must have the same streams (same codecs, same time base, etc.)" The concat demuxer is good for when you are trying to avoid re-encoding, but that is not possible with inputs of various formats.

You are providing inputs with arbitrary attributes. Use concat filter instead:

ffmpeg -i a.webm -i b.webm -i c.m4a -i d.webm -filter_complex "[0:a][1:a][2:a][3:a]concat=n=4:a=1:v=0[a]" -map "[a]" output.mp3

Note from the concat filter documentation: "The filtering system will automatically select a common sample format, sample rate, and channel layout for audio streams."

If you want to manually select the sample rate and channel layout, so you know exactly what you will get, add the aformat filter :

ffmpeg -i a.webm -i b.webm -i c.m4a -i d.webm -filter_complex "[0:a]aformat=r=44100:cl=stereo[a0];[1:a]aformat=r=44100:cl=stereo[a1];[2:a]aformat=r=44100:cl=stereo[a2];[3:a]aformat=r=44100:cl=stereo[a3];[a0][a1][a2][a3]concat=n=4:a=1:v=0[a]" -map "[a]" output.mp3