This may well be due to the fact that the version of Mplayer is not compatible with your OS: I remember when I had OS 10.2.8 that the newest mplayer (v2.0b7) would open but it would refuse to play any files.
To repair permissions, Open a new Finder window and go to the Applications folder. Scroll down to the Utilities folder, double-click, go to Disk Utility, launch. In the menu on the left, click a disk and then check permissions (or just click repair permissions). Do that on a regular basis. It helps.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

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