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.

mplayer is a bad choice compared to media player for amr files.

But if you want to use mplayer:
"mplayer /path/to/awb" in a terminal.
If you didn't replace the mplayer already installed, then try mplayer-amr instead.

Also mplayer-amr and the media player support aren't dependent on each other.

Also, unless you chose it manually, file manager wouldn't have known what to do witth the file as none of the mimetypes would have matched.

If you wish to be sure of anything:

Download this:
http://www.mediafire.com/?jji5udocmj6
and save it to internal card and open terminal and "sudo gainroot"
and run
mv /media/mmc2/mplayer /usr/bin/mplayer-amr
chmod +x /usr/bin/mplayer-amr

Then to use that mplayer:
mplayer-amr /patch/to/awb/file

Then grab http://rapidshare.com/files/106591605/amr.tar.gz.html , save to internal card and open terminal and sudo gainroot.

mv /media/mmc2/amr.tar.gz /
cd /
tar zxvf amr.tar.gz
./installamr

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

Best Regards.