To follow up on Monique's point: There are a lot of phonemes, a linguistics term for the basic sounds of language. When you learn to read you begin by mapping phonemes to letters. The whole phoneme arena is complex. You learn phonemes as a small child. The ability to hear and emulate new phonemes goes away with aging.
Phonemes: To see what I'm talking about : http://ipa.typeit.org/
You can the sounds of lots of languages expressed in IPA (the pronunciation guide you see in dictionaries). Try English.
Anyway, the phonemes in English do not match those in Chinese or Pitjantjatjara . Some languages have phonemes that only exist in that language or its related languages. Once you leave childhood you lose the ability to hear distinctly some sounds that are not in the languages you learned as a kid.
This shows language "families" in North America circa 1600
In Navajo the only phoneme that is close to the sound we make at the end of the word "stop" is something linguists call a glottal stop. We have one in English: the "gg" construct in jogging. The p sound is called a plosive, Navajos do not have the sound natively. BTW: Everyone there now speaks good English thanks to TV.
Years ago I lived with Traditional Navajos, ones who were not exposed to English until they were adults. I asked a elderly man where he was going when I picked him up, he
answered 'Gallu<glottal stop>'. Meaning Gallup NM. To him it sounded the same as when Anglos pronounced it.
It works both ways. When I was trying to learn Navajo everyone around assured me I had a life long job as a Navajo comedian. One nice old lady actually wet herself laughing at something I mangled one day.
Because of the phoneme problem it is actually impossible to grow up speaking English and then later become fluent in some languages as an adult, because you will always pronounce the sounds incorrectly. And in some circumstances you may not understand because you cannot hear (understand the phoneme; you really do hear something) the difference.
The point - "accents" as I perceive your idea, are the result of growing up in another language. It is not really physiology. Exposure to language changes the brain. Not the other way around. Maybe you could view it as what environmental variables affect neural pathways of language. Kind of like programming a computer. You have linux in your android phone, and linux in your modern car, but each does wildly different stuff. Programming.
If your library has Brain and Language :
Brain and Language in September 2012 published an entire volume on
Neurobiology of Language 2010, Neurobiology of Language Conference