features:

• persistent media player (floatable or placed in the topbar) - allows for audio files to be stored in an internal playlist and played as desired (sourced from files plugin, external sites (mp3, flv, mpg etc - including youtube and others). includes an option to switch to video mode to watch in a lightbox or perhaps even in a popup or new tab.
• persistent here means that one way or another, the media does not stop playing as the site is navigated. the best way i know of to do this is to load all internal pages via ajax and retain the player and other elements as only loading once. this has the knock-on benefit of greatly reducing server load and bandwidth.
• all pages will load faster.
• media seen via the river can be played without leaving the river.. so less navigation is involved and the river becomes the 'hub' it is best suited to be.

i started a thread on this here: http://community.elgg.org/discussion/view/1435010/html5-ajax-persistent-audio-and-elgg
in some senses this is such a large change that it is perhaps a core design issue.. though since i have not coded it, i'm not 100% sure.. maybe a plugin can be made without altering the core?