sure it works with mplayer, but AIUI, you want to use it with MAFW (just like OMP and vanilla Media Player).

If you don't care which backend you're using, try smplayer - it's frontend for mplayer that have GUI options for virtually everything that mplayer can handle, so I'm pretty sure it allows stretching to screen and disabling aspect ratio, not to mention handling multi-audio sources correctly. Once, I even played a music files via USB sound card connected to N900 via it (by using ALSA output instead of pulseaudio - try it with MAFW as long as you want, sadly, it won't work, due to undefined bugs in our pulseaudio implementations).

/Estel