I believe you can only get mplayer and sib from maemo extra-testing. In app manager, tap on the menu bar (top of screen) and application catalog. You will notice about three repository (basically links to different types of apps). Maemo Extra-testing is probably disabled or not there so you need to add it. FOllow step below. Save and get out of that screen. App manager should reload. Go into multimedia to find mplayer and sib. Install them. Mplayer is a command line app only so that's why SIB is needed. KMplayer is also suitable gui for mplayer.

WARNING: These apps are beta and not stable so exercise caution when using them. I don't recommend installing too many of these apps.

Catalog Name: Maemo Extra-testing
Web Add: http://repository.maemo.org/extras-testing
distribution: fremantle
components: free non-free

HIT SAVE and update your app manager to see new apps. REMEMBER, you are warned. Test at your own risk.