In fact I have succeeded to convert FLV to MP3:
D:\tmp\ffmpeg-20121005-git-d9dfe9a-win64-static\ffmpeg-20121005-git-d9dfe9a-win
4-static\bin>ffmpeg.exe -i a.flv -acodec mp3 a.mp3
ffmpeg version N-45080-gd9dfe9a Copyright (c) 2000-2012 the FFmpeg developers
  built on Oct  5 2012 16:49:01 with gcc 4.7.1 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-pthreads --enable-run
ime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libass
-enable-libcelt --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-
ibfreetype --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libopen
peg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libthe
ra --enable-libutvideo --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-l
bvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --en
ble-zlib
  libavutil      51. 73.102 / 51. 73.102
  libavcodec     54. 63.100 / 54. 63.100
  libavformat    54. 29.105 / 54. 29.105
  libavdevice    54.  3.100 / 54.  3.100
  libavfilter     3. 19.102 /  3. 19.102
  libswscale      2.  1.101 /  2.  1.101
  libswresample   0. 16.100 /  0. 16.100
  libpostproc    52.  1.100 / 52.  1.100
Input #0, flv, from 'a.flv':
  Metadata:
    metadatacreator : iku
    hasKeyframes    : true
    hasVideo        : true
    hasAudio        : true
    hasMetadata     : true
    canSeekToEnd    : false
    datasize        : 16906383
    videosize       : 14558526
    audiosize       : 2270465
    lasttimestamp   : 530
    lastkeyframetimestamp: 529
    lastkeyframelocation: 16893721
  Duration: 00:08:49.73, start: 0.000000, bitrate: 255 kb/s
    Stream #0:0: Video: h264 (Main), yuv420p, 448x336 [SAR 1:1 DAR 4:3], 218 kb
s, 15 tbr, 1k tbn, 30 tbc
    Stream #0:1: Audio: aac, 44100 Hz, stereo, s16, 32 kb/s
File 'a.mp3' already exists. Overwrite ? [y/N] y
Output #0, mp3, to 'a.mp3':
  Metadata:
    metadatacreator : iku
    hasKeyframes    : true
    hasVideo        : true
    hasAudio        : true
    hasMetadata     : true
    canSeekToEnd    : false
    datasize        : 16906383
    videosize       : 14558526
    audiosize       : 2270465
    lasttimestamp   : 530
    lastkeyframetimestamp: 529
    lastkeyframelocation: 16893721
    TSSE            : Lavf54.29.105
    Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16
Stream mapping:
  Stream #0:1 -> #0:0 (aac -> libmp3lame)
Press [q] to stop, [?] for help
size=    8279kB time=00:08:49.78 bitrate= 128.0kbits/s
video:0kB audio:8278kB subtitle:0 global headers:0kB muxing overhead 0.006842%
But I failed to convert FLV to MP4. Why is the encoder 'mp4' unknown? What's more, how can I find the codecs which are already supported by my FFmpeg?
D:\tmp\ffmpeg-20121005-git-d9dfe9a-win64-static\ffmpeg-20121005-git-d9dfe9a-win6
4-static\bin>ffmpeg.exe -i a.flv -acodec mp4 aa.mp4
ffmpeg version N-45080-gd9dfe9a Copyright (c) 2000-2012 the FFmpeg developers
  built on Oct  5 2012 16:49:01 with gcc 4.7.1 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-pthreads --enable-runt
ime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libass -
-enable-libcelt --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-l
ibfreetype --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libopenj
peg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheo
ra --enable-libutvideo --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-li
bvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --ena
ble-zlib
  libavutil      51. 73.102 / 51. 73.102
  libavcodec     54. 63.100 / 54. 63.100
  libavformat    54. 29.105 / 54. 29.105
  libavdevice    54.  3.100 / 54.  3.100
  libavfilter     3. 19.102 /  3. 19.102
  libswscale      2.  1.101 /  2.  1.101
  libswresample   0. 16.100 /  0. 16.100
  libpostproc    52.  1.100 / 52.  1.100
Input #0, flv, from 'a.flv':
  Metadata:
    metadatacreator : iku
    hasKeyframes    : true
    hasVideo        : true
    hasAudio        : true
    hasMetadata     : true
    canSeekToEnd    : false
    datasize        : 16906383
    videosize       : 14558526
    audiosize       : 2270465
    lasttimestamp   : 530
    lastkeyframetimestamp: 529
    lastkeyframelocation: 16893721
  Duration: 00:08:49.73, start: 0.000000, bitrate: 255 kb/s
    Stream #0:0: Video: h264 (Main), yuv420p, 448x336 [SAR 1:1 DAR 4:3], 218 kb/
s, 15 tbr, 1k tbn, 30 tbc
    Stream #0:1: Audio: aac, 44100 Hz, stereo, s16, 32 kb/s
Unknown encoder 'mp4'