avconv -i test.mka -i Spirited\ Away\ 720p\ HDTV\ \(1\).mka
avconv version v0.8-1337-gd293e34, Copyright (c) 2000-2012 the Libav developers
  built on Apr  6 2012 23:31:19 with gcc 4.6.3
[matroska,webm @ 0x29cf760] Estimating duration from bitrate, this may be inaccurate
Input #0, matroska,webm, from 'test.mka':
  Metadata:
    ENCODER         : Lavf54.2.0
  Duration: 02:04:31.07, start: 0.000000, bitrate: 32 kb/s
    Stream #0.0(eng): Audio: mp3, 48000 Hz, 2 channels, s16, 32 kb/s (default)
[matroska,webm @ 0x29d2a40] Estimating duration from bitrate, this may be inaccurate
Input #1, matroska,webm, from 'Spirited Away 720p HDTV (1).mka':
  Duration: 02:04:31.09, start: 0.000000, bitrate: N/A
    Stream #1.0(eng): Audio: aac, 48000 Hz, stereo, s16
At least one output file must be specified