% ffmpeg -i 20150113_180642_B7E7/20150113_180642_B7E7.mkv -f webm -acodec libvorbis a.webm
ffmpeg version 2.5.2 Copyright (c) 2000-2014 the FFmpeg developers
  built on Dec 25 2014 00:04:42 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
  configuration: --extra-libs=-ldl --prefix=/opt/ffmpeg --enable-avresample --disable-debug --enable-nonfree --enable-gpl --enable-version3 --enable-libpulse --enable-libopencore-amrnb --enable-libopencore-amrwb --disable-decoder=amrnb --disable-decoder=amrwb --enable-libx264 --enable-libx265 --enable-libfdk-aac --enable-libvorbis --enable-libmp3lame --enable-libopus --enable-libvpx --enable-libspeex --enable-libass --enable-avisynth --enable-libsoxr --enable-libxvid --enable-libvo-aacenc
  libavutil      54. 15.100 / 54. 15.100
  libavcodec     56. 13.100 / 56. 13.100
  libavformat    56. 15.102 / 56. 15.102
  libavdevice    56.  3.100 / 56.  3.100
  libavfilter     5.  2.103 /  5.  2.103
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
[matroska,webm @ 0x323e0e0] Could not find codec parameters for stream 1 (Video: h264, none, 1280x720): unspecified pixel format
Consider increasing the value for the 'analyzeduration' and 'probesize' options
Input #0, matroska,webm, from '20150113_180642_B7E7/20150113_180642_B7E7.mkv':
  Metadata:
    encoder         : GStreamer plugin version 0.10.31
    creation_time   : 2015-01-13 18:06:58
  Duration: 00:00:22.66, start: 0.000000, bitrate: 81 kb/s
    Stream #0:0(eng): Audio: aac (LC), 16000 Hz, mono, fltp (default)
    Metadata:
      title           : Audio
    Stream #0:1(eng): Video: h264, none, 1280x720, SAR 1:1 DAR 16:9, 3 fps, 1k tbr, 1k tbn, 2k tbc (default)
    Metadata:
      title           : Video
File 'a.webm' already exists. Overwrite ? [y/N] y
[buffer @ 0x324fd60] Unable to parse option value "-1" as pixel format
    Last message repeated 1 times
[buffer @ 0x324fd60] Error setting option pix_fmt to value -1.
[graph 0 input from stream 0:1 @ 0x3233640] Error applying options to the filter.
Error opening filters!