As someone who used to work in closed captioning and did a little subtitling, I can tell you that it's not uncommon for the studios to simply re-use old closed caption files done years ago for VHS and have them converted into subtitles for DVD. It's cheaper. And that's where the sound effects in the files come from... a new subtitle file has no sound effects, because they're really intended for a hearing audience, not the hearing impaired.
Many years ago, it was believed by most in the captioning industry that people weren't capable of reading fast enough to get every single word of speech in a film or television show. After conducting tests with the hearing impaired, it was determined that, yes, deaf people can read really fast, so it's okay to caption every word spoken onscreen.
So these older caption files (generally pre-2000, depending on the company), are heavily edited. There may still be some captioning houses out there that do edited files; I don't know.
So when a home video company decides they don't want to spend the money for a whole new subtitle job, which for a two-hour movie would involve about 24 hours of labor, they can have the old closed caption file converted to subtitles for less than half the price. Many of these are now apparently being ported over from DVD to blu-ray.
Some editing is still occasionally necessary, though. Sometimes, when someone is talking REALLY fast, or several speakers overlap, it's just not possible to put every word in.
Regarding errors in captioning/subtitling: Very often the production company does NOT provide a script to the captioning house. Even when they do, sometimes even the cutting continuity script is wrong in spots... those are transcribed by outside vendors, too.
For television shows, very often what is captioned is not the final cut... audio overdubs have yet to be added. This is why you have lines in the captioning for TV shows that are very different from what is being said or are not even in the show at all. Worst thing is, the production company usually has a hissy fit and blames the captioning company for this, until they are informed that they never sent out a final cut for captioning. Everything is on such fast turnaround for TV... frequently a change is made to a show literally the day before it airs.
