Pynchon's name...

[Doug Millison]

...is pronounced Pyn-CHON, with the accent on the second syllable, in the
Books-on-Tape edition of M&D that was released shortly after the novel was
published.  I rented and listened to it at the time.  (Highly recommended.)
I assume that the producers of this audio version of the novel made their
choice of pronunciation with input from Pynchon, but don't know this for a
fact.