title: error: opus in MP4 support is experimental, add '-strict -2' if you want to use it
body: <p>I am trying to convert some MKV to MP4 with ffmpeg, and I keep getting the error below. Any thoughts?</p>
  <p>I have installed ffmpeg and installed:</p>
  <pre><code>sudo apt install -y libopus-dev libmp3lame-dev libfdk-aac-dev libvpx-dev libx264-dev yasm libass-dev libtheora-dev libvorbis-dev mercurial cmake
  </code></pre>
  <p>Command I ran with output:</p>
  <pre><code>ffmpeg -i 'video.mkv' -codec copy 'video.mp4' -strict -2 -y
  ffmpeg version 4.0.2-2 Copyright (c) 2000-2018 the FFmpeg developers
    built with gcc 8 (Ubuntu 8.2.0-7ubuntu1)
    configuration: --prefix=/usr --extra-version=2 --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-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared
  
  [mp4 @ 0x560e85326100] track 1: codec frame size is not set
  [mp4 @ 0x560e85326100] opus in MP4 support is experimental, add '-strict -2' if you want to use it.
  Could not write header for output file #0 (incorrect codec parameters ?): Experimental feature
  Stream mapping:
    Stream #0:0 -&gt; #0:0 (copy)
    Stream #0:1 -&gt; #0:1 (copy)
      Last message repeated 1 times
  </code></pre>
  <p>I also tried adding <code>-strict -2</code> to no avail.</p>