ffmpeg -i /spare/in.mp4a -acodec libfdk_aac -flags +qscale -global_quality 3  /spare/out.mp4
 libavutil    50. 44. 0 / 50. 44. 0
  libavcodec   52.123. 0 / 52.123. 0
  libavformat  52.111. 0 / 52.111. 0
  libavdevice  52.  6. 0 / 52.  6. 0
  libavfilter   1. 81. 0 /  1. 81. 0
  libswscale    0. 14. 1 /  0. 14. 1
  libpostproc  51.  2. 0 / 51.  2. 0
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/spare/in.mp4a':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42isom3gp63g2a3gp4
    creation_time   : 2008-01-26 05:36:13
  Duration: 00:04:27.86, start: 0.000000, bitrate: 64 kb/s
    Stream #0.0(und): Audio: aac, 44100 Hz, stereo, s16, 64 kb/s
    Metadata:
      creation_time   : 2008-01-26 05:36:13
[libfdk_aac @ 0x1fb2660] Value 1.000000 for parameter 'signaling' out of range
[libfdk_aac @ 0x1fc16e0] VBR quality 0 out of range, should be 1-5
[libfdk_aac @ 0x1fc16e0] Note, the VBR setting is unsupported and only works with some parameter combinations
Output #0, mp4, to '/spare/out.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42isom3gp63g2a3gp4
    creation_time   : 2008-01-26 05:36:13
    encoder         : Lavf52.111.0
    Stream #0.0(und): Audio: libfaac, 44100 Hz, stereo, s16
    Metadata:
      creation_time   : 2008-01-26 05:36:13
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop, [?] for help
size=    3476kB time=00:04:27.86 bitrate= 106.3kbits/s
video:0kB audio:3385kB global headers:0kB muxing overhead 2.682648%