The activity in the textbook was the pronunciation of the present simple third person singular ‘s’. ‘So what?’, most native English might say. ‘It’s pronounced /s/.’ But no. Consider ‘lifts’, ‘pulls’ and ‘pushes’. The last sound of ‘lifts’ is indeed /s/, but last sound of ‘pulls’ is actually /z/ and the last two sounds of ‘pushes’ are /əz/ (for most native English speakers).
(I started drafting an explanation about exactly why this happens, but that is actually irrelevant to my point. See here for a discussion of voicing and unvoicing with regard to plural nouns – the principles are broadly similar. Because there are far more voiced sounds in English than unvoiced, far more present simple third person singular verbs (and plural nouns) end with /z/ than /s/.)
The paper textbook has four examples of each group and the electronic version also has recordings. As I played the recordings, I noticed that in three of the examples for /z/, the pronunciation was actually /s/. In technical terms, the /z/ was being devoiced.
I started wondering how common this is, and whether it is increasing in prevalence, but without access to technical research, I will probably never know.
In any case, devoicing a /z/ doesn’t affect (or at least rarely affects) the meaning, particularly in context. While a devoiced /z/ on ‘sees’ (/siz/ > /sis/) makes it sound like ‘cease’, those two words never occur in the same context.
I said to the students, ‘The pronunciation is less important. What is more important is that you say ‘s/z’ at all.’ Even this may not be true. Many varieties of English do not use present simple third person singular ‘s’; speakers of those varieties say ‘He see’ rather than ‘He sees’. Unfortunately for everyone, those varieties carry less prestige in the English speaking world and the standard and/or prestige varieties use ‘s’. A lot would be gained and nothing would be lost if we simply abolished present simple third person singular ‘s’, but I don’t see that happening soon.