$ ffmpeg -i se_t630.amr se_t630.mp3
ffmpeg version 2.3.1 Copyright (c) 2000-2014 the FFmpeg developers
  built on Aug  5 2014 23:26:16 with Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/2.3.1 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-nonfree --enable-hardcoded-tables --enable-avresample --enable-vda --cc=clang --host-cflags= --host-ldflags= --enable-libx264 --enable-libfaac --enable-libmp3lame --enable-libxvid --enable-libopencore-amrnb --enable-libopencore-amrwb
  libavutil      52. 92.100 / 52. 92.100
  libavcodec     55. 69.100 / 55. 69.100
  libavformat    55. 48.100 / 55. 48.100
  libavdevice    55. 13.102 / 55. 13.102
  libavfilter     4. 11.100 /  4. 11.100
  libavresample   1.  3.  0 /  1.  3.  0
  libswscale      2.  6.100 /  2.  6.100
  libswresample   0. 19.100 /  0. 19.100
  libpostproc    52.  3.100 / 52.  3.100
[amr @ 0x7fecc2021800] Estimating duration from bitrate, this may be inaccurate
Input #0, amr, from 'se_t630.amr':
  Duration: 00:00:05.57, bitrate: 5 kb/s
    Stream #0:0: Audio: amr_nb (samr / 0x726D6173), 8000 Hz, mono, flt, 5 kb/s
Output #0, mp3, to 'se_t630.mp3':
  Metadata:
    TSSE            : Lavf55.48.100
    Stream #0:0: Audio: mp3 (libmp3lame), 8000 Hz, mono, fltp
    Metadata:
      encoder         : Lavc55.69.100 libmp3lame
Stream mapping:
  Stream #0:0 -> #0:0 (amr_nb (amrnb) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
se_t630.amr: Input/output error
size=       6kB time=00:00:05.62 bitrate=   8.6kbits/s
video:0kB audio:6kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 4.531250%