Yesterday, I finished some work on my callcenter system and I found problems with converting WAV (G729) to MP3. My pioneering work bring me some sweet fruits and I will share this solutions for you.

We need this packages:

libgsm

lamemp3

ffmpeg

Here is fast script:

wget http://howto-pages.org/ffmpeg/libgsm-1.0.13.tar.bz2
wget http://downloads.sourceforge.net/project/lame/lame/3.98.2/lame-398-2.tar.gz?use_mirror=dfn
wget http://ffmpeg.org/releases/ffmpeg-0.5.tar.bz2
tar -xjvf libgsm-1.0.13.tar.bz2.
cd libgsm-1.0.13
make && make install
cd ..
tar -zxvf lame-398-2.tar.gz
cd lame-398-2c
./configure
make && make install
cd ..
tar -xjvf ffmpeg-0.5.tar.bz2
cd ffmpeg-0.5
./configure --enable-gpl --enable-libgsm --enable-libmp3lame
make && make install

(Lot of people are facing error on loading shared libraries libavdevice.so.52, solution is:
export LD_LIBRARY_PATH=/usr/local/lib/).

You can check, if compiling are allright: ffmpeg –formats

Results will be like this:

FFmpeg version 0.5, Copyright (c) 2000-2009 Fabrice Bellard, et al.
configuration: --enable-gpl --enable-libgsm --enable-libmp3lame
libavutil     49.15. 0 / 49.15. 0
libavcodec    52.20. 0 / 52.20. 0
libavformat   52.31. 0 / 52.31. 0
libavdevice   52. 1. 0 / 52. 1. 0
built on Sep  9 2009 11:58:57, gcc: 4.1.2 20071124 (Red Hat 4.1.2-42)

After compiling and installing you can try convert: ffmpeg -i somefile.WAV someanotherfile.mp3

Some another install istructions for wav (not g729) are here: http://www.thiscoolsite.com/?p=73

Are you satisfied?