$FFMPEG_HOME/bin/ffmpeg  -i rec_1452.3gpp -acodec libmp3lame -ar 22050 audio_only_test.mp3

--stdout 

ffmpeg version N-30956-g81ef892, Copyright (c) 2000-2011 the FFmpeg developers
  built on Jun 22 2011 17:37:00 with gcc 4.5.3
...
  libpostproc  51.  2. 0 / 51.  2. 0
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'rec_1452.3gpp':
  Metadata:
    major_brand     : 3gp4
    minor_version   : 0
    compatible_brands: isom3gp4
  Duration: 00:01:27.44, start: 0.000000, bitrate: 5 kb/s
    Stream #0.0(eng): Audio: amrnb, 8000 Hz, 1 channels, flt, 5 kb/s
Incompatible sample format 'flt' for codec 'libmp3lame', auto-selecting format 's16'
[mp3 @ 000000000025FF80] Unsupported sample rate.
Output #0, mp3, to 'audio_only_test.mp3':
  Metadata:
    major_brand     : 3gp4
    minor_version   : 0
    compatible_brands: isom3gp4
    TSSE            : Lavf53.4.0
    Stream #0.0(eng): Audio: libmp3lame, 22050 Hz, 1 channels, s16, 64 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop, [?] for help
Warning, using s16 intermediate sample format for resampling
size=     684kB time=00:01:27.51 bitrate=  64.0kbits/s
video:0kB audio:684kB global headers:0kB muxing overhead 0.017855%

example for 3gpp file aaptured on phone coverting to more generic format/codec by using 'ffmpeg'

audio conversion 3gpp to mp3

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