It seems that youngsters a third of my age seem to be able to memorise this and type it in without thinking, but I need this put somewhere my lazy neurons can find it.
vlc -vvv "input.flv" --no-sout-video --play-and-exit --sout="#transcode{acodec=mp3,ab=320,channels=2}:std{access=file,mux=raw,dst="output.mp3}"

...and don't forget to double-check that VLC has write permission for the target directory (it is very cautious). Yes yes yes, should be Linux, should be ogg etc. etc. but sometimes my hands are tied.