Load an named icon called "gtk-media-pause", and it won't be themed because the themes use "media-pause", or "stock-media-pause" instead. The same applies to the other GTK_STOCK_MEDIA_* icons in gtk+.
It is the themes responsibility to cover both gtk stock and icon-naming spec names. One could consider making icon-naming-utils create symlinks for the stock gtk media icons, but that is somewhat questionable, since gtk has rtl versions too, and we don't want to theme just the ltr versions. It is probably better to have ion themes explicitly cover stock icons. I have recently posted a script to gnome-themes-list which can help ensuring good coverage, and I have used it to improve the Bluecurve coverage. Please take this upstream.