From the Clan Menzies Society web site:

Well that depends. In the area were the Clan originated it is definitely pronounced “Mingus”. However the Clan Society recognises that those of the name Menzies have spread far beyond the limits of Strathtay where not everyone is familiar with local pronunciation and is perfectly happy to accept those who prefer to pronounce their name in a more phonetic style. Though, David Menzies of Menzies (the Clan Chief) when refered to as “Men-zies of Men-zies” is reputed to have responded that there is no such person.

--http://www.menzies.org/faq.htm
Celtic name pronounciation are generally changed by Spanish speakers -- for instance, there was an Irishman named Hugo O'Connor who served with the Spanish Army on the frontier in Pimeria Alta (modern-day Arizona) -- the Spanish could not pronounce the O'Connor, so it was changed to "Oconor".

It sure does look like the Menzies tartan! :mrgreen:

Cheers,

Todd