$ ffmpeg -i test.mp3 -c:a libfdk_aac -map 0:a:0 -vbr 4 -f mp4 -
ffmpeg version 2.3.git Copyright (c) 2000-2014 the FFmpeg developers
  built on Sep 11 2014 15:21:28 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
  configuration: --prefix=/home/ubuntu/ffmpeg_build --extra-cflags=-I/home/ubuntu/ffmpeg_build/include --extra-ldflags=-L/home/ubuntu/ffmpeg_build/lib --bindir=/home/ubuntu/bin --enable-gpl --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-nonfree
  libavutil      54.  7.100 / 54.  7.100
  libavcodec     56.  1.100 / 56.  1.100
  libavformat    56.  4.101 / 56.  4.101
  libavdevice    56.  0.100 / 56.  0.100
  libavfilter     5.  1.100 /  5.  1.100
  libswscale      3.  0.100 /  3.  0.100
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  0.100 / 53.  0.100
[mp3 @ 0x29c1e60] Estimating duration from bitrate, this may be inaccurate
Input #0, mp3, from 'test.mp3':
  Metadata:
    title           : Track 10
    album           : Commercial DEMO
    artist          : Jon Novi
    genre           : Other
    track           : 10/17
    encoder         : Audiograbber 1.83.01, Fraunhofer acm, 56 kBit/s, 22,050 Hz, Stereo, HQ = off
  Duration: 00:00:57.92, start: 0.000000, bitrate: 56 kb/s
    Stream #0:0: Audio: mp3, 22050 Hz, stereo, s16p, 56 kb/s
[libfdk_aac @ 0x29d2b20] Note, the VBR setting is unsupported and only works with some parameter combinations
[mp4 @ 0x29c42e0] muxer does not support non seekable output
Output #0, mp4, to 'pipe:':
  Metadata:
    title           : Track 10
    album           : Commercial DEMO
    artist          : Jon Novi
    genre           : Other
    track           : 10/17
    encoder         : Lavf56.4.101
    Stream #0:0: Audio: aac (libfdk_aac) ([64][0][0][0] / 0x0040), 22050 Hz, stereo, s16
    Metadata:
      encoder         : Lavc56.1.100 libfdk_aac
Stream mapping:
  Stream #0:0 -> #0:0 (mp3 (native) -> aac (libfdk_aac))
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument