Some of the examples, like the girl saying at the end "so... yeah" are not the same. She is making a summary, a conclusion (without saying it), but it is a concluding "so". It alludes to whatever else she said before, and then at the end,
saying "so (then here would be the summary of what was said before the concluding statement)... yeah!"
I've noticed it mostly in young people who are usually university/college graduates, who perhaps want to give an air of importance to whatever they are saying.
The Perry Mason example is also different, because he begins his remarks with a so, but he is then referring to whatever facts were presented to the jury, etc and he says, "so, members of the jury, after carefully blah blah blah)
What this thread is about, is people who begin an answer to a question with a "so", but are not making a summary or concluding remark. I've hear this a lot on NPR, on DemocracyNow! also, on British radio/TV etc.
There is a show on NPR called Le Show, where the host, Harry Shearer, has begun a segment called "the so's of the week", where he plays many examples. Many times the interviewee answers several questions in a row, all beginning with so. Or with right! so
I find it annoying.