How to add captions file to video podcast? I'm starting a tutorial video podcast and have recorded a couple of episodes. 

I've captioned them, and have the captions file in .srt format. However I haven't yet found a way to embed that file in the .mp4 video file that makes the captions optional for podcast viewers.

How can the captions be included with the podcast, so that viewers can turn them on only if they want them, but they will be off by default? I'm most interested in getting this working in the iTunes podcast section and the iOS Podcast app to start with (although a universal solution that works with all podcast clients would be great).

UPDATE 3rd April 2014:

No luck yet I'm afraid with solving this. Until something better comes along, in the podcast feed, for each &lt;item&gt;, I set the &lt;link&gt; element to a URL that hosts a version of the video with optional captions (captions are off by default), and towards the end of the body of the &lt;description&gt; element I repeat the same URL mentioning alongside it that subtitles are available there. See http://a.webdevbreak.com/episodes if you'd like to see an example.
Ordinarily, I'd create an Adobe AIR or AIR-for-iOS app to solve this problem. You'd just sychronize your captions to cue points in the video and make 'em visible or invisible with a programmed-in button. You could easily have multiple languages in this way.

Are iTunes podcasts just flat-out mp4 movies?