The additional 'r' after the hyphen prevented nix-env from recognizing the SVN
revision number as a version'ish kind of thing; thus nix-env would never update
an installed mplayer in any user profile.
In other words, to get this particular update, you must remove the old
expression by running
nix-env -e mplayer-r33472
..., and then install the new one:
nix-env -i mplayer
svn path=/nixpkgs/trunk/; revision=31785
9b740035a4