This article is from the Pratchett Newsgroups FAQ, by Orin Thomas email@example.com with numerous contributions by others.
Terry himself pronounces it to rhyme with "Mince pinned" and in Eric,
Rincewind meets his ancestor whose translated name means "Washer
[Rinser?] of Winds". Evidence would then tend to point to this
pronunciation (like the pronunciations in the Discworld Game)