The box CPU has not enough horsepower for that (and no hardware floating point unity is available)
Video transcoding, for example, is impossible.
Some audio transcoding is possible, it depends on the original and desired format. Some libraries use integer computations (e.g. "tremor", for ogg) and it should possible to transcode for that format.
Transcoding to mp2/3 is impossible, see the lame/twolame packages. ffmpeg is also available.
Generally it depends on the software, mediatomb uses/depends on ffmpeg, that is a kind of swiss-knife for transcoding, but it might not use all possible ffmpeg transcoding, don't know.
In any case, you have to setup transcoding first on a desktop PC, then try to replicate the setup in the box and report what is missing (providing the necessary command lines used and errors reported)
Cheers