How can I concatenate 2 videos with audio using ffmpeg? [duplicate] - ffmpeg

Question

This question already has an answer here:
How to concatenate videos in ffmpeg with different attributes?
(1 answer)
Closed 2 years ago.
I tried this command:
ffmpeg -f concat -i mylist.txt -c copy output.mp4
But output.mp4 has no audio.
mylist.txt contains:
file '1.mp4'
file '2.mp4'
Information about 1.mp4 and 2.mp4:
C:\Users\Admin\OneDrive\Desktop\New folder>ffmpeg -n -i 1.mp4 -i 2.mp4
ffmpeg version git-2020-05-23-26b4509 Copyright (c) 2000-2020 the FFmpeg developers
built with gcc 9.3.1 (GCC) 20200523
configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libsrt --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --disable-w32threads --enable-libmfx --enable-ffnvcodec --enable-cuda-llvm --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt --enable-amf
libavutil 56. 48.100 / 56. 48.100
libavcodec 58. 87.101 / 58. 87.101
libavformat 58. 43.100 / 58. 43.100
libavdevice 58. 9.103 / 58. 9.103
libavfilter 7. 83.100 / 7. 83.100
libswscale 5. 6.101 / 5. 6.101
libswresample 3. 6.100 / 3. 6.100
libpostproc 55. 6.100 / 55. 6.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '1.mp4':
Metadata:
major_brand : mp42
minor_version : 0
compatible_brands: mp41isom
creation_time : 2020-05-31T08:12:56.000000Z
Duration: 00:00:05.93, start: 0.000000, bitrate: 186 kb/s
Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 184 kb/s, 30 fps, 30 tbr, 30k tbn, 60 tbc (default)
Metadata:
creation_time : 2020-05-31T08:21:56.000000Z
handler_name : VideoHandler
encoder : AVC Coding
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from '2.mp4':
Metadata:
major_brand : mp42
minor_version : 0
compatible_brands: mp41isom
creation_time : 2020-06-01T03:32:58.000000Z
Duration: 00:00:12.23, start: 0.000000, bitrate: 1909 kb/s
Stream #1:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 1708 kb/s, 30 fps, 30 tbr, 30k tbn, 60 tbc (default)
Metadata:
creation_time : 2020-06-01T04:17:17.000000Z
handler_name : VideoHandler
encoder : AVC Coding
Stream #1:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 196 kb/s (default)
Metadata:
creation_time : 2020-06-01T04:17:17.000000Z
handler_name : SoundHandler
At least one output file must be specified

Answer 1

For concatenation to work:
According to the concat demuxer documentation:
Each input must have the same number of video and audio streams.
Each video and audio stream must have the same attributes.
Each input must have the same video and audio format.
What is wrong with your inputs
1.mp4 has no audio. 2.mp4 has audio.
1.mp4 needs audio to be able to concatenate to 2.mp4.
Solution
Add silent audio to 1.mp4 using anullsrc filter:
ffmpeg -i input.mp4 -f lavfi -i anullsrc=channel_layout=stereo:sample_rate=48000 -map 0:v -map 1:a -c:v copy -c:a aac -shortest 1b.mp4
Then update mylist.txt to:
file '1b.mp4'
file '2.mp4'
Finally run your ffmpeg command to concatenate:
ffmpeg -f concat -i mylist.txt -c copy -movflags +faststart output.mp4