The html edit box strips it out - it has a list of approved codes. Look in the tinymce folder:

\mod\tinymce\views\default\input

it has a view called longtext.php this sets up the editor control

and see these pages for documentation:

http://wiki.moxiecode.com/index.php/TinyMCE:Plugins/media

http://tinymce.moxiecode.com/

consider having your own longtext.php view, perhaps with a different view if isadminloggedin() is set

Consider using this mod to edit so that default tinymce is not changed:

http://community.elgg.org/pg/plugins/markbeachill/read/268808/tinymce326-plugin-wrapper-for-tinymce-version-326

Hope this helps

Mark