Do we know how Greek dialects sounded?

Question

To some extent, we know how sounds varied between ancient Greek dialects: the Aeolians lost rough breathings but preserved digamma, for example, while the Attics changed many of their long alphas into etas.

But apart from alphabet changes, do we have any evidence of how dialectal pronunciation differed? For example, even if all dialects used theta to write the first letter in θεος, I doubt they all pronounced that theta the same way: at some point it shifted from a plosive to a fricative, and that can't have happened everywhere at the same time.

Writing could potentially give evidence: for example, if X dialect inscriptions tend to substitute hypsilon for iota, we can assume /y/ and /i/ were merging. Similarly, if a comedian has a character from Y island say "ουάναξ", then Y dialect probably preserved the digamma even if they didn't write it. However, I don't know of any actual examples of this.

Answer 1

The nice thing about Greek dialect inscriptions is that there was little in the way of standardized orthography: spellings seem to closely track the local pronunciation (or sometimes apparently the pronunciation of the scribe, who wasn't always necessarily a local). So the answer is yes, we know quite a bit; this is the main reason we know anything about Greek dialect differences in the first place. The dialect handbooks, e.g. Buck's Greek Dialects, are full of examples.

For an extreme case, look at Boeotian, which is full of spellings that look bizarre from an Attic point of view but seem to have been chosen for phonetic accuracy. These mostly involve vowels; here are some examples:

1. The sound that Attic writes οι appears in Boeotian as υ (e.g. ϝυκια for Attic οἰκία). The explanation seems to be that [oi] has been monophthongized to some sort of front rounded vowel like [ø] or [y]. Since upsilon was used for [y] in Attic, the Boeotians decided to use it for this similar sound.
2. The sound that Attic writes υ appears in Boeotian as ου (e.g. ουπερ for Attic ὑπέρ). This clearly stands for [u], which is the original value of this phoneme, fronted in Attic. After dentals this is sometimes even spelled ιου (e.g. ονιουμα = ὄνομα), indicating some kind of palatalization or diphthongization.
3. Other monophthongizations include η for αι (e.g. κη for Attic καί) and ι for ει, foreshadowing already in the fifth century BC the much later phenomenon of Koine iotacism.

Of course, the Greeks didn't use IPA and there's a level of phonetic resolution that is difficult to capture with the Greek alphabet or any other, so our knowledge of exactly how the dialects sounded is necessarily incomplete. Some dialects actually invented new letters to represent sounds specific to them, e.g. Arcadian "tsan", and we can only make educated guesses at what those stood for. Your example of theta is interesting: the Greek alphabet had no sign for [θ], so we can't necessarily tell whether the use of theta in a given case stands for the aspirated plosive or the fricative, but in this case the fricative pronunciation of some dialects was marked enough that it left traces in literature: a Spartan character in Aristophanes' Lysistrata swears Ναὶ τὼ σιώ for Ναὶ τὼ θεώ "by the Twin Gods". Presumably the sigma was the playwright's way of spelling [θ].