Supporting music from a stream is not the same as playing .mp3 from a folder of files, there's a whole bunch of file i/o stuff that the latter has to deal with whereas the former doesn't.
I'm not suggesting its hard to implement, however what it does do is open up a whole set of contingencies to maintain platform security. Playback isn't the issue, its everything around it that'll be keeping the OS-team thinking.
Playback of video files is the same, but with the added complication of pressure from media companies to include DRM functionality. PS3 video playback supports Cinavia for instance, a technology that no doubt will have been pushed on them as a neccessity by Sony's own media arm, and will be an additional licensing cost to be factored in.