'Excuse me' is pronounced with a /z/ because 'excuse' is a verb in this context. If you look in a good dictionary - preferably one with an audio function - you'll see that the verb 'excuse' has a /z/ sound at the end, while the noun 'excuse' has an /s/ sound. For example: "He excuses himself" = /ziz/ "I have no excuses" = /siz/

ZZZZ... ;)