need help using ffmpeg to "concat" multiple audio files (webm, mp4) to make one longer MP3 of audio - audio

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