Marv, I am far from a multimedia guy and I usually use VLC because it uses it's own libraries (the ones provided by it's installation) for everything. That makes it almost foolproof.
But I have played a lot with MEncoder and a bit with MPlayer in the past as well as FFMPEG.
Have you opened gnome-mplayer from a sakura terminal window using "gnome-mplayer -v"?
If you have done so, you have discovered that by default it uses MPlayer v1.
You can go into gnome-mplayer settings and choose which MPlayer binary to use. Do that and setup MPlayer2's binary.
Then run it again with -v from the terminal and either hope that it works with MPlayer2 or that at least the terminal window gives you a hint
But I am quite sure you have already done all that so let's hope someone savvy chimes in here.