3gp, 2gp , mp4 conversion issues with ffmpeg

ffmpeg accepts only encoding 3gp , 2gp , mp4 files (E- means encoding D-deconding)
ffmpeg -formats | grep 3gp

E 3g2 3gp2 format
E 3gp 3gp format
DE amr 3gpp amr file format
D mov,mp4,m4a,3gp,3g2,mj2 QuickTime/MPEG4/Motion JPEG 2000 format
ffmpeg -formats | grep mp4

E mp4 mp4 format
E psp psp mp4 format


so it is needed that mencoder to be used to decode the
3gp,2gp,mp4 files



mencoder example.3g2 -nosound -ovc lavc -lavcopts
vcodec=mpeg4 -o example.avi
mplayer -vo null -ao pcm -af
resample=8000,volume=+4db:sc example.3g2
ffmpeg -i example.avi -i audiodump.wav example.flv