I have asserted that the weird thing about the 27th letter of the alphabet, the apostrophe, is that it never actually represents a sound (see also here). Eric Smith has pointed out to me that if you take the e of fishes to represent the second vowel phoneme of that word, then you might want to say that in fish's the apostrophe represents that same vowel phoneme. Maybe that would be best. But I had been thinking of 's as being a representation of the whole genitive-suffix morpheme, which shares all its pronunciation with the plural-suffix morpheme, but not as giving a sound-by-sound spelling out of it. Notice that the genitive of cat is cat's, where there is no second vowel sound for the apostrophe to represent. It is possible that Eric's view is better, which would mean that the apostrophe sometimes represents nothing (as in don't or he'll) but sometimes represents a reduced unstressed vowel ([ə] or [ɪ]). But notice (important point!), this isn't a question about what the facts are. Statements about which letters in a spelling symbolized which sounds are theoretical proposals. The issue here is about whether Eric defends a better theory of the sound-spelling relationship than I was assuming. It's not about what the facts of English are, because Eric and I agree on those.
A comparable example arises with a word like guilt (pronounced [ɡɪlt]). Does the gu part symbolize [ɡ] and the i symbolize [ɪ]? Or does the g symbolize [ɡ] leaving the ui part to symbolize [ɪ]? We can't examine spectrograms or manuscripts to find out. The question is about how to set up the best theory of the sound-spelling relationship. Relevant evidence might include the spelling of gilt or guard or guest, the spelling of the second syllable of biscuit or linguist, and who knows what else. It's by no means a trivial puzzle.
There are some brief relevant remarks about this in The Cambridge Grammar of the English Language, pages 17-18, and a lot more sound and spelling stuff in Chapter 18 on inflection. We cite an important book on the topic: John D. Mountford, An Insight into English Spelling (London: Hodder and Stoughton Educational, 1998).
[In comments the mm part symbolizes [m], and in closed it is probably best to see the s as symbolizing [z], but you might be able to argue that it's the se sequence. Get the idea? Don't answer that.]