Thank you for your advice Mike Keitz,  Sam Haile, Graham Daniel, Steve
Baldwin and Jason Wolfson.  I suppose it makes a lot of sense to investigate
the ISD chips a little bit more closely.  The things that I need to find out
about these chips are if their memories are non-volatile and whether it is
easy to make duplicates of pre-recorded chips for production purpouses.  The
ones I've seen so far does not meet either criteria.

I once saw a Texas Instruments chip once which meets both these criteria.
It's a one-time programmable chip which allows you to store a number of
speech messages (128 seconds worth).  The disadvantage is that its one time
programmable, but the advantage is that it's completely non-volatile.
Decisions... decisions hey?  :)

Rgds
Werner