mplayer is currently optimised for DivX-style AVIs containing MP3 audio. AAC encoded audio, often found with .mp4 files, will not work well with mplayer.

They should play back well in the built-in player. Your converter options boil down to:

* Nokia Internet Tablet Video Converter - MP4 video, AAC audio, .mp4 container. Media Player and Canola.
* everything else (...) - DivX/Xvid/MP4-style video, MP3 audio, .avi container. All media players.