Quayle and Tiax were played strictly for laughs, and the "helium" tinged voice fits that (although I think that helium is an exaggeration, since both of them have voices that fit the range of adult males, albeit the high end of that range).
Jan Janson was also a joker, but his story had a serious side to it. His voice is on the high end of the range for adult males, but nobody would mistake it for a child's voice or a woman's voice. Helium is definitely an exaggeration here too.
I liked all three of these gnomes, and brought them all along in various BG groups. Tiax for the insane one liners, and Jan for overall funny. The only Faerun gnome that I have contempt for is the one in Neverwinter Nights 2, who was just horribly written.
