My search was indeed too quick. I stand corrected. Captions/subtitles seem like a very good idea for accessibility, but in addition to that I think that an alt attribute would still be appropriate for browsers that can't display the media at all. The alt is a replacement for an external element that cannot be displayed at all for whatever reason. It replaces the element. That's why I would suggest alt attributes for objects, embeds, frames, etc. too.
Remco
