iframes can now be whitelisted through the allowed_iframes Site Setting. It accepts a list of iframe src domain prefixes that discourse can safely allow in posts. The whitelist-iframe plugin is no longer required.
What I have done is make a plugin that calls the Discourse.Markdown.whiteListIframe function. The plugin is very simple, The hard part is that you have to supply regular expressions to match any URLs you want to white-list. The code is here. https://github.com/scossar/whitelist-iframe
You can test any URLs you wish to whitelist in your web browser’s console window. For example, if you open the browser’s console from a Discourse forum and then enter the following line, you should be able to embed iframes from Freesound.
window.Discourse.Markdown.whiteListIframe(/^(https?:)\/\/www\.freesound\.org\/embed\/sound\/iframe\/.+/i);
There may be an easier way to do this. 