Thanks for the file, checked it. It contains AAC audio track. Currently MPlayer does not use DSP codec for AAC and uses its own code for it. The AAC decoder from MPlayer is not optimized for ARM currently, moreover it uses floating point math in default configuration. It is possible to configure it to use fixed point math, but fixed point decoder is much less stable and it even does not support one of the AAC variants. As N800 has vfp, I configured AAC decoder to use floating point math, but it is still too slow to play this file.

It is all about priorities. Surely, AAC playback can be improved (maybe using DSP decoder), but it requires quite a lot of work. In the mean time fixing or improving something else might be better. Right now MPlayer is optimized for mpeg4 video (and variants such as divx,xvid,flv,etc.) and mp3 audio on Internet Tablets. Also there will be always media formats (mostly proprietary and closed source) which are supported by built-in player, but very poorly supported or not supported at all in mplayer, real media files for example. It is generally better to avoid such media files if possible, as they are an evil thing and may bite you in the future (think about the possibility of the company behind this format getting out of business and upgrade to some new hardware/os with no codecs to support this format any longer).

As for the bugs, their fixes and stabilization of MPlayer on N800. We need to decide what is important for us, what needs to be fixed ASAP, and what can wait a bit longer. Hint: bugtracker at MPlayer garage project page works and the bugs which are reported there actually get fixed