At the moment this functionality is still unavailable for current versions of the application. Anyway, our developers have a plan to implement the ability to include video/audio clips for email templates in future versions of the Creatio application (not earlier than version 7.18.).
At the moment as a workaround, we can suggest adding the Image block to the email template. For the image can be used the screenshot from the needed video (before adding it to the template, it would be better if you compress the picture, it can be done via different online image compression tools, for example, https://compresspng.com/). After the image compressed and added to the template, please attach the short YouTube video link to the image.