Maybe what @silverpill was trying to say was — ActivityPub already has support for them (articles, videos, podcasts):
https://www.w3.org/TR/activitystreams-vocabulary/#dfn-article
https://www.w3.org/TR/activitystreams-vocabulary/#dfn-video
https://www.w3.org/TR/activitystreams-vocabulary/#dfn-audio
But, you are correct, that — their software would have to output ActivityPub data when called with "Accept: application/activity+json".