Growing up in Stockton, Calif., in what she described as a “very, very, very, very, very loud” Cambodian American family, Samantha Lamb remembers Christmas gatherings during which her younger brother, Anthony Veasna So, quietly sat at his computer while cousins, aunts, uncles and other relatives played games and talked over one another. “It can be perceived as him receding into the background,” she said in a video interview last week, “but actually, he was taking notes on us, writing his next piece.” Lamb can see her family members in virtually all of the young, old, immigrant, Americanized, queer, straight, hard-working, irresponsible, male and female Khmer American characters who appear in “Afterparties,” So’s collection of nine short.