I’ve read throught all the replies of this post and I’m not completely satisfied with the proposed solutions. As a foreword, I must say that we should all keep in mind that not all Markdown users are accomplished developers or technician. So, we should make our possible to keep the language as simple and concise as possible. Example usage are social networks. The federated ones in particular (diaspora, Ganggo, SocialHome, Friendica).
I don’t think using ![]() for all media contents with media type guess is a good solution. One presented corner case is the the MP4 which can be audio only or video. This means the user should specify the media type anyway. Plus, it is an established fact that ![]() is for images. Extend it to support all media type could disturb the user (it’s a different use case).
Using generic attributes to disambiguate seems not an good idea either. While it is a nice feature, I don’t think it should be mandatory for embedding medias. It’s yet another syntax logic to learn and not all users know what an attribute or a mime type is.
Generic directives are the worst to my eyes. We should keep in mind that audio and video are words that make sens in french and english but not in swaili. While it works for the :youtube[title]{vid=09jf3ow9jfw} example, because Youtube is a brand, :audio[title]{vid=09jf3ow9jfw} makes no sens in russian.
What I propose is a new syntax for audios and a new one for videos, close to the one for images: #[]() for videos and ~[]() for audios. it is elegant and sementically right. # symbolises a screen while ~ symbolises a sound wave.