Hmm, now I'm confused. Installing mplayer amr shouldn't have affected the media player as media player wouldn't work with just those libraries on it's own.
Well, I provided mplayer as that is the first thing I thought of. But when I saw the limitations, I decided to do it for the media player.
I would still recommend installing the media player amr support as it provides proper gstreamer support (the media player backend) and shows the amr files in the library.
So mediaplayer will be the default to open awb files. mplayer will only open amr from the command line, when you invoke it manually. media player is much better for awb files than mplayer.