Firstly, with regard to spelling: "Today it is no longer considered incorrect to use either form (Jones's or Jones') ".
Secondly, ncsu ties together pronunciation and spelling to a limited degree:
Spelling conventions are in some sense an attempt to replicate
pronunciation. When style manuals make pronouncements on spelling,
they imply that the presence of an extra "s" means that most educated
speakers customarily pronounce that sound. The reason that this system
cannot simply be reversed to produce pronunciation rules is that style
manuals are directed at written language, and they are thus also
interested in visual consistency and eliminating visual distractions.
This is in answer to the question:
In American English, how are words that end in "s's" pronounced? For
example, if Jones is pronounced as "Jonz," how is Jones's pronounced?
And if the above were written to end with the apostrophe (Jones'), is
the final "s" silent? Listening to the radio, I notice that many
announcers seem to treat the final "s" as silent; but I feel the final
"s" should be pronounced as a separate syllable, even in cases where
the word is written to end with the apostrophe only.
This article says that you can choose between the addition of the bare apostrophe or the apostrophe s in these cases, but that this then dictates how the resultant should be pronounced. Yet another rule dropped from on high.
This article from Grammar and Style in British English implies that there is a rigorous connection between pronunciation and written form; it recommends that you should decide whether one pronunciation sounds more reasonable, and choose the spelling to correspond as closely as possible. For Jones' / Jones's and Wales' / Wales's, it implies there is no obviously better choice:
Where possessive nouns ending in s make a harsh ziz sound, the option
is available of using an apostrophe without an additional s. Thus –
Jones’s house is the one at the end of the street
may instead be written
Jones’ house is the one at the end of the street.
Gareth Edwards was one of Wales’s (or Wales’) finest rugby players.
But the traditional practice is to retain the additional s
(Jones's house) unless the resulting pronunciation would be
particularly ungainly –
Moses’ tablets / Cervantes’ Don Quixote / Jesus’ disciples
Ultimately, one has to decide whether one is sufficiently pro-'Joneses' to read out Jones' as 'Joneses', etc. In a sense, 'only read the form Jones's as 'Joneses' is prescriptivist. One faces a very similar problem when quoting a passage with alternative spellings one doesn't use, when quoting passages using different spellings, when reading out 'Wagner conducted the Wagner piece' ...