AMR

samr	This is an audio codec. The name to use at the command line is samr.

sawb	This is an audio codec. The name to use at the command line is sawb.

Tutorial

To stream amr-nb with vlc to mobile-phone eg. next setting works (tested on few nokia & sony ericsson phones)

transcode{vcodec=H263,width=128,height=96,vb=28,fps="12.5",acodec=samr,ab="5.25",samplerate=8000,
channels=1,venc=ffmpeg{keyint=6,strict=1,strict-rc=1}}:rtp{mp4a-latm,port-video=2244,port-audio=2242,dst=1
27.0.0.1,ttl=12,sdp=\"file:///usr/local/movies/mobile.sdp\"}

It seems that mp4a-latm somehow affects on audio-output on amr also. AMR-NB requires 8000 samplerate and only 1 channel, also if you choose incorrect audio-bitrate, vlc gives you unknown codec or similar error.