Why not use automatic Google Captions or YouTube Captions?
First of all it's great you are even thinking about making your videos more accessible. By providing Captions to your media you open your audience up to not only Deaf and Hard of Hearing but also to the foreign language group learning English. YouTube has tried to make this process easy. In fact, on November 9, 2009, Google video and YouTube introduced caption tracks - namely, machine-generated automatic captions. However, sad to say, this new "accommodation" has actually not brought to fruition the true intent of its creator's hopes and dreams who incidentally is Deaf himself. There is still a lot of work to be done in this area, mainly because language is so dynamic and there are so many accents that a machine just can't pick up on and caption. So now there's a whole genre of memes circulating on the web, now called "Automatic Caption Fails". I came across this site while researching and I thought it illustrated the problem perfectly, but there are lots more out there.
http://captionfail.tumblr.com/
Of course technology is advancing and we look forward to the innovations of the future but, in the meantime I think we should stick to humans doing the captioning.
What do you think? Do you have a Caption Fail you would like to share?
Reference: Google Official Blog
Of course I must say we use humans in the captioning of our videos! ;)