ffmpeg says "Unknown encoder 'mp3'"

Question

ffmpeg version 1.1.3 Copyright (c) 2000-2013 the FFmpeg developers
  built on Feb 25 2013 23:22:43 with gcc 4.1.2 (GCC) 20080704 (Red Hat 4.1.2-44)
  configuration: --enable-static --disable-ffplay --disable-ffserver --enable-gpl --enable-libxvid --enable-libx264 --enable-postproc --enable-libvpx --enable-libtheora
  libavutil      52. 13.100 / 52. 13.100
  libavcodec     54. 86.100 / 54. 86.100
  libavformat    54. 59.106 / 54. 59.106
  libavdevice    54.  3.102 / 54.  3.102
  libavfilter     3. 32.100 /  3. 32.100
  libswscale      2.  1.103 /  2.  1.103
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/usr/local/httpd-2.2.25/htdocs/mvix/resources/testsp/mov01.mov':
  Metadata:
    major_brand     : qt  
    minor_version   : 537199360
    compatible_brands: qt  
    creation_time   : 2011-04-25 22:07:51
    comment         : Encoded and delivered by apple.com/trailers/
    comment-eng     : Encoded and delivered by apple.com/trailers/
    copyright       : 漏 2011 Paramount Pictures. All Rights Reserved
    copyright-eng   : 漏 2011 Paramount Pictures. All Rights Reserved
    title           : Transformers: Dark of the Moon
    title-eng       : Transformers: Dark of the Moon
    timecode        : 00:00:00:00
  Duration: 00:02:27.86, start: 0.000000, bitrate: 5353 kb/s
    Stream #0:0(eng): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1280x532, 5215 kb/s, 23.98 fps, 23.98 tbr, 2997 tbn, 5994 tbc
    Metadata:
      creation_time   : 2011-04-25 22:07:51
      handler_name    : Apple Alias Data Handler
    Stream #0:1(eng): Audio: aac (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 132 kb/s
    Metadata:
      creation_time   : 2011-04-25 22:07:51
      handler_name    : Apple Alias Data Handler
    Stream #0:2(eng): Data: none (tmcd / 0x64636D74)
    Metadata:
      creation_time   : 2011-04-25 22:07:51
      handler_name    : Apple Alias Data Handler
      timecode        : 00:00:00:00
Please use -b:a or -b:v, -b is ambiguous
Unknown encoder 'mp3'

Solution

As the error message says, there is no encoder named mp3 in ffmpeg. There are various encoders for MP3. In ffmpeg you'd use libmp3lame. See also the MP3 encoding guide.

Your build must be configured with --enable-libmp3lame. See How to Compile FFmpeg on CentOS or simply use a Linux build of ffmpeg. Either method will provide a build that supports libmp3lame.

You can check the available encoders with ffmpeg -encoders.

Also, check your syntax. While you haven't provided your actual command (you should always provide the command), note that -b as an option alone is ambiguous. If you want to specify video or audio bitrate, use -b:v or -b:a, respectively.