We recently started embedding subtitles in m4v files destined for iPods, iPhones, etc. and have learned a few things along the way that I thought I would share here, especially since subtitling and closed captioning documentation for Apple devices seems to be hard to come by.
1) The software we use to embed the subtitles into the m4v files wants a SubRip file (.srt) that includes a number for each caption. For example:
00:00:00,000 –> 00:00:03,125
00:00:03,125 –> 00:00:04,625
And so on. Without the numbering, the embedding will fail.
2) When checking the embedded movie in Quicktime player on a computer, only 2 attribute tags render correctly. The <i> italic code and the <u> underline codes work, but the bold and font color tags seem to be ignored. We will check these on the iPad to see if they display properly there, but I’m betting they won’t. My assumption is that at this time Apple is supporting a subset of the expanded SRT spec that VLC uses.