YouTube continues to have support for both machine-based audio transcriptions and manually created captions / transcriptions.
It does not appear to be a function of the Flash vs HTML5 players.
Video owners can explicitly disable the machine transcription:
You can't delete the auto-captions, but you can hide them from
To prevent viewers from requesting auto-captions in the player, on the
Captions and Subtitles pane, uncheck the box next to the track called
English: Machine Transcription. If you change your mind, click the
checkbox again to enable auto-captions.
Also, it appears that YouTube uses some heuristic to determine which videos receive a machine transcription. I was not able to find confirmation of this.
Using Google Chrome stable (v18) on Ubuntu, I was able to enable audio transcriptions by selecting the (CC) icon in both the Flash and the HTML5 interfaces on this video: http://www.youtube.com/watch?v=vRyaW1DBMF4
The following video does not have a machine transcription available, though it is unclear if that is the decision of the video owner or a result of the theoretical YouTube heuristic resulting in a low confidence of speech being present: http://www.youtube.com/watch?v=w68qZ8JvBds
In addition to the machine transcription, there are two other options for video owners:
You can upload a caption file, with specific time coding and text to
display as captions.
Video owners can let YouTube machine caption their video, then
download, edit, and upload the polished transcription. YouTube will
then display the captions at the appropriate time, without need for
manual time coding.
You can opt in or out of the HTML5 trial at any time by visiting http://www.youtube.com/html5