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I am using following command but it is giving wrong output.

ffmpeg -i video.webm -i audio.opus -c:v copy -c:a aac -strict experimental output.mp4

Output:

ffmpeg version 1.2.6-7:1.2.6-1~trusty1 Copyright (c) 2000-2014 the FFmpeg developers

  built on Apr 26 2014 18:52:58 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)

  configuration: --arch=amd64 --disable-stripping --enable-avresample --enable-pthreads --enable-runtime-cpudetect --extra-version='7:1.2.6-1~trusty1' --libdir=/usr/lib/x86_64-linux-gnu --prefix=/usr --enable-bzlib --enable-libdc1394 --enable-libfreetype --enable-frei0r --enable-gnutls --enable-libgsm --enable-libmp3lame --enable-librtmp --enable-libopencv --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-vaapi --enable-vdpau --enable-libvorbis --enable-libvpx --enable-zlib --enable-gpl --enable-postproc --enable-libcdio --enable-x11grab --enable-libx264 --shlibdir=/usr/lib/x86_64-linux-gnu --enable-shared --disable-static

  libavutil      52. 18.100 / 52. 18.100

  libavcodec     54. 92.100 / 54. 92.100

  libavformat    54. 63.104 / 54. 63.104

  libavdevice    53.  5.103 / 53.  5.103

  libavfilter     3. 42.103 /  3. 42.103

  libswscale      2.  2.100 /  2.  2.100

  libswresample   0. 17.102 /  0. 17.102

  libpostproc    52.  2.100 / 52.  2.100

Input #0, matroska,webm, from 'video.webm':

  Duration: 00:00:04.92, start: 0.000000, bitrate: 243 kb/s

    Stream #0:0: Video: vp8, yuv420p, 640x480, SAR 1:1 DAR 4:3, 18 fps, 18 tbr, 1k tbn, 1k tbc (default)

Input #1, ogg, from 'audio.opus':

  Duration: 00:00:05.16, start: 0.000000, bitrate: 32 kb/s

    Stream #1:0: Audio: opus, 48000 Hz, stereo, s16

[mp4 @ 0x647940] track 0: could not find tag, codec not currently supported in container

Output #0, mp4, to 'output.mp4':

  Metadata:

    encoder         : Lavf54.63.104

    Stream #0:0: Video: vp8, yuv420p, 640x480 [SAR 1:1 DAR 4:3], q=2-31, 18 fps, 90k tbn, 1k tbc (default)

    Stream #0:1: Audio: aac ([64][0][0][0] / 0x0040), 48000 Hz, stereo, fltp, 128 kb/s

Stream mapping:

  Stream #0:0 -> #0:0 (copy)

  Stream #1:0 -> #0:1 (libopus -> aac)

Could not write header for output file #0 (incorrect codec parameters ?): Operation not permitted

WebM containers typically contain VP8 or VP9 video. The MP4 container format however does not support VP8 or VP9 video, at least according to the registration authority.

In your command you are trying to copy the video bitstream, and obviously it will fail:

codec not currently supported in container

You have two options:

1. Re-code the video to H.264 using -c:v libx264.


2. Choose another output container like Matroska (.mkv), but note that this will not be compatible for HTML5 video.

See also this MDN reference on supported HTML video formats and this Super User post on how to convert video for HTML5: What bunch of ffmpeg scripts do I need to get HTML5-compatible "Video for everybody"?

ffmpeg -i video.webm -i audio.opus -c:v copy -c:a aac -strict experimental output.mp4

it should look something like this.

ffmpeg -i audio.opus -i vid.webm  -c:v copy -c:a opus -strict experimental output6.webm

replace aac with opus and the two files around with opus being in front(i think), it should work.

A simple command with copy option with the conversion from webm to mp4 doesn't work because of difference in a codec which we don't touch during conversion. It is like simply changing the extension of the input file.
To make sure the output is played in H.264 players/televisions use

ffmpeg -y -i "input_video.webm" -i "input_audio.webm" "output_file.mp4"  -c:v libx264 -c:a aac

Note that the execution will be very slow and add -preset veryfast depending on your quality needs.
Ref: https://addpipe.com/blog/converting-webm-to-mp4-with-ffmpeg/