I don't know, unless you missed the instructions to uninstall the old QT4, before you install QT4 4.6.2. There may be some residue left over that can cause trouble, if you didn't. I am also using MPlayer from the mplayer-SVN-r30994-OS2-4.4.2.zip package (at HOBBES), which actually does the work.
FWIW, I usually start SMPlayer by double clicking an associated file, but using the menu also works.