- Compile and install vsound.
- Install RealPlayer 11 for Linux. In Tools > Preferences > Hardware, make sure it's using OSS as the Audio driver (it has to be using /dev/dsp for output; ALSA seems to use /dev/snd/foo).
- On the command line, run vsound -v --file=OUTPUT.wav realplay INPUT.amr, replacing INPUT and OUTPUT with the appropriate filenames.
AMR files are often produced by mobile phone audio recordings, but AMR includes patented codecs so can't be compiled into GStreamer or FFMPEG, hence this awkwardness.
If you've compiled your own version of GStreamer and so have the amrnbparse and amrndec pipelines, you can probably use this amr2wav script.
Found Mobile Media Converter too, which has an easy GUI for converting between AMR/3GP and MP3/MPEG.