S...Z ?
                S  ?  Z?      Z ?   
   S ?   Z?
Why does the "s" in words like "surprise" or "wise" sound like /z/ ?

Humm...  no idea.  However...

When the letter "s" is after 
a vowel
a second "s"
a voiced consonant 

it is always pronounced as a /z/.

Some words that are spelled Z in the United States, 
are spelled S in the United Kingdom.


Advise
Advertise
Business
Busy
Deposit
Disaster
Desert
Dessert
Deserve
Design
Desire
Easy
Enterprise
Franchise
Husband
Improvise
Merchandise
Music
Noisy
Observe
Otherwise
Positive
Position
Present
President
Prison
Reason
Represent
Resident
Resist
Reservation
Resolve
Result
Revise
Rise
Rose
Season
Sunrise
Surprise
Thursday
Transition
Visit
Wise
Sampling of words with a consonant and end with "s" that sounds like /z/.

Bags
Beds
Boys
Cats
Dogs
Pens
Rugs
Words
Sampling of words with two "s" and end with an "s" that sounds like /z/.

Addresses
Bosses
Classes
Glasses
Kisses
Losses
Misses
Passes
Sampling of words that end with an "s" after a vowel, that sounds like /z/.

Clothes
Beaches
Becomes
Glasses
Heroes
Moves
Noses
Packages