Is there a biological basis for different accents?
No. Accent is purely environmental. The ability to pick up the unique sounds of a language or variation of a language ceases around puberty: http://en.wikipedia.org/wiki/Language_acquisition

This is why adults learning a second language have trouble distinguishing the unique sounds of the second language. For learners of English, the "th" sounds and the difference between "l" and "r" can be difficult. For languages based on Sanskrit, there are a variety of aspirated/unaspirated consonants somewhere between "b" and "v" that English-speakers have a hard time distinguishing, much less imitating.