When I was a kid, my voice got more nasal when I felt like I was saying something smart. I also remember reading a book, one of Harry Harrison's Stainless Steel Rat books, describing the protagonist's voice as "growing nasal" as he described the cool part of his plan. I don't experience this phenomenon anymore, and I've never heard anyone else talk about it. If you search around for why nerd characters stereotypically have nasal voices, the primary hypotheses are allergies, asthma, poor posture.