Re: Why Southern accent in the US is different from Standard American accent?
The term 'Southern Accent' is a very broad brush, as others have said, and somewhat subjective as well.
Generally, the closer you live to an area, the more distinguished your ear in terms of pinpointing accents. While somebody from outside the US may have trouble distinguishing one Southern accent from another, somebody who lives in the Southern US can likely tell you within a less than a hundred miles exactly where somebody lives (or grew up), based on their accent. Some dialects and accents are specific enough to narrow to a single town or county.
This is true of any country, and any language, by the way, not just the U.S. For example, I could probably accurately differentiate between an Irish, Scottish, and English accent, but I couldn't tell you what part of Ireland, Scotland, or England aside from some haphazard guessing, unless it was something extremely distingtive like a Cockney accent. I can't accurately pick out a Welsh accent from an English one.
For a broad range of accent samples from not only various US Southern accents, but accross the United States and across the world, spend some time listening to samples on the I
nternational Dialects of English Archive (IDEA).
Click on the 'Dialects and Accents' Tab, then progressively narrow down your geographic region. The neat thing about the archive is is also factors into account other factors such as age, gender, first language, ethnicity, education, and other factors such as amount of time in that region, outside influences (i.e. living abroad), etc.
For your "Southern Accent", pick some samples from Virginia, Tennessee, Georgia, Mississippi, Alabama, and Texas, and you'll quickly see that there is not such thing as one "Southern" accent.
After listening to a few of those, navigate to Australia and listen to some samples from various regions in Australia, and I think you'll easily be able to hear they really sound nothing similar.
There are generally two parts to each sample - an excerpt reading from a standard passage, and then conversational prompts. You'll note that some peoples' accents change noticeably from the reading prompts to the conversational prompts. Factors such as education and professionalism tend to kick in during reading, and people may adopt more neutral or standardized accent, while then tend to slip back into their stronger regional dialect when talking of childhood memories, etc. Sometimes, there are phonetic transcriptions of both speech samples.
Really quite fascinating stuff, in my opinion. The downside is that they really need a lot more samples of different people from the same regions.
A couple of related videos:
Amy Walker - 21 accents. Again, I can't speak for the other variants, but her American accents are spot on.
Here, she does only A
merican accents. Around the 1:30 mark, she discusses some differences in Southern US accents. Note that she comments that these are all generalizations.
Sara - gibberish accents. Interesting, because it focuses on sounds, not words. She's not saying anything coherent in any of her various accents, just gibberish but manages to sound pretty authentic to me on ones I'm familiar with.
Guide to British Isles accents. Perhaps some of our members from across the pond can comment on the veracity of his observations - I can hear the differences, but can't of course comment on the accuracy.