Cant Create Audio File From MP4

[Davy]

I cannot use VLC to convert my mp4 video files to simple audio files.
I follow the Media/Convert wizard to convert to mp3, flac or aac but in each case generate an output file the same size as the input video file.

Conversion to mp3 gives a .ps file (does not play if I rename it to .mp3)
Conversion to flac gives a header with zero track length
Conversion to aac gives a warning that VLC cannot determine what mux.

Is there something simple I am missing? In each case it seems that VLC is trying to create a muxed container.

'dump raw input' (don't know what that is) is unchecked.
Encapsulation set to 'raw'


Davy