Hello Marlo,
I have no proven answer to your query, but have certainly observed
the same phenomenon, and thought about it some.  All I can offer is
my unsubstantiated theory.  We do know of course that it is not
merely a time-related change, because live-taken specimens with good
transparency will retain their transparency for many years.  So, it
is apparently some external and fairly vigorous agent that causes the
rapid transformation from transparent to opaque white.  When you say
"dead collected", you presumably mean "beach collected", probably in
the "beach drift" or "shell hash" that is deposited near the high
tide line - and that, I believe, is the key.  A shell doesn't just
ride in on a wave and get gently deposited in the drift line.  Before
it ever reaches that final resting place, it has been driven up and
down the beach many times by the ebb and flow of the surf, each time
tumbling along among millions of rapidly moving sand grains.  Sand
grains (in most localities) are largely quartz and other dense
minerals, far harder material than the calcium salts of the shell.
Consequently, the shell gets innumerable tiny (probably mostly
microscopic) scratches all over it.  The result (I theorize) is the
same as you would get if you sandpapered a sheet of plexiglass, or
even a pane of glass - a change from glossy and transparent to dull,
opaque, and apparently white.  "Frosted glass" is made by two
separate processes - acid etching and/or sand blasting.  I suspect
that what we have here are "frosted shells" resulting from sand
blasting.
Hmm - that raises an interesting question - in areas where the sand
is mostly of coral origin, are the microshells generally in better
condition than in areas where the sand is primarily of rock origin?
Anybody know?
 
Paul M.