Bonjour Elizabeth,

The word 'plus' can be pronounced differently : [ply-s], [ply] or [ply-z]

In the case above, 'plus' from 'de plus' is pronounced [ply-s] as in [ss] as it means 'furthermore', i.e. an addition. 

The following link, from one of our education partners, explains the three different types of pronunciation: pronunciation-plus

I hope this is helpful.

Bonne journée !