After trying for a while to get mplayer working on 3.5.1, I finally got it. I installed XMMS already which I believe setup a lot of the symlinks for me.
I used the following packages from xmms-embedded.sourceforge.net/feed/oz32
mplayer_20030614-10_armv4l-strongarm.ipk
libvorbisidec1_20030614-1_armv4l-strongarm.ipk
Extract them manually with:
ar -x mplayer_20030614-10_armv4l-strongarm.ipk
tar zxvf data.tar.gz
Copy the usr/bin/mplayer to /mnt/card/usr/bin/mplayer (or appropriate destination)
and etc/mplayer to /mnt/card/etc/mplayer (again, XMMS already has symlinks created at root, I believe)
Then you need usr/lib/libvorbisidec.so.1.0.2 out of the libvorbisidec package. (otherwise it'll complain about an ogg format and you cant play anything, even non-ogg)
You dont need to bother with the symlinks out of that directory.
Replace the old (newer) libvorbisidec.so.1.0.2 with the one from the package.
I had to remove /root/.mplayer/config and /root/.mplayer/codecs.conf to finish it up.
Mplayer is working in XMMS for me now, just like it did in 3.3.6pre1.