When multimedia filters are enabled some media players (at least YouTube and Vimeo) try to transform links to those sites into iframes so, if you insert a link to those sites on a forum post with that filter enabled, the link gets converted to an iframe and that iframe content isn't shown on email notification content because email server/client sanitizes the iframes, so that content isn't shown at all, not the link nor the iframe.

Steps to replicate this with YouTube player (it also happens with Vimeo and the YouTube setting of the VideoJS player):

• Enable the multimedia plugin
• Enable the YouTube media player and put it the first of the list to ensure it gets applied
• Create a test course and enrol a user into it
• Create a forum with forced or auto subscription
• Add a new topic and insert a link to some YouTube video in the message body
• Check that on Moodle that link gets converted to an iframe
• Check that on the email notification sent to the user the link and the iframe aren't shown at all

Most modern e-mail providers usually sanitize iframes, so to avoid this happening I think that multimedia filters that use iframes may shouldn't be applied to notifications sent by email.

I doubt if it should be done also with players using HTML5 <video> and <audio> tags like VideoJS as they are still supported only by a few email clients.