Pronunciation of onomatopoeia, pharmacopoeia, etc

Solution 1:

OED gives

Pronunciation: Brit. /ˌɒnəmatəˈpiːə/ , U.S. /ˈˌɑnəˌmɑdəˈpiə/ , /ˈˌɑnəˌmædəˈpiə/ Inflections: Plural onomatopoeias, unchanged.

Etymology: < classical Latin onomatopoeia the making of words < Hellenistic Greek ὀνοματοποιία < ancient Greek ὀνοματο- onomato- comb. form + -ποιία -poeia comb. form. Compare Middle French, French onomatopée imitative formation of a word (1585), a word formed in this way (1768; 1665 as omomatopeia : compare sense 1b), Italian onomatopea (1560; a1498 as onomatopia), Old Occitan onomothopeya (c1330), Spanish onomatopeya (1450 as onomatopeia).

N.E.D. (1903) gives the pronunciation as (onǫ:mătopī·yă, ǫ:nŏmă-) /əʊˌnɒmətəʊˈpiːjə/ /ˌɒnəʊmæt-/ .

First use in English was:

1553 T. Wilson Arte of Rhetorique iii. f. 92v, A worde makinge called of the Grecians Onomatopeia is when we make wordes of oure owne mynde, suche as be deriued from the nature of thinges.

They don't give any pronunciation history farther back than 1903.

It's instructive that Late Latin and the Romance languages usually pronounced this word's ending with an /e/ instead of an /i/:

• Latin onomatopoeia /-peia/ from earlier /-pojia/
• French onomatopée /-pe:/
• Italian onomatopea /-pea/ but also onomatopia /-pia/
• Old Occitan onomothopeya /-peja/
• Spanish onomatopeya & onomatopeia /-peja/

While the Greek underwent a vowel shift of the ending to Modern Greek /-pi'ía/. This reminded me of English's own Great Vowel Shift (GVS). If the first use in English was as far back as 1553, then the vowel may have been subject to the GVS. In particular:

Middle English [ɛː] raised to [eː] and then to modern English [iː] (as in beak).
Middle English [eː] raised to Modern English [iː] (as in feet).

The above would result in an original /-peja/ becoming the modern /pija/. So, it seems possible that the original English borrowing from Latin/Greek had an /e/ and that it has changed to /i/ since then.

This is detective work and reasoning, not citation, so take it for what it's worth.

Solution 2:

I agree with everything in Mark Beadles' answer. I just wanted to address one point from the original question that wasn't covered yet.

It would not be standard to simplify “poeia” to “pia,” and I haven’t found any evidence of this occurring except as an occasional spelling error. In general, the digraph “oe” used to transliterate Greek "οι" can be simplified to “e.” This would yield the simplified spelling “onomatopeia.” Wiktionary does list this as a “US spelling of onomatopoeia,” but I haven’t found it listed by any other dictionaries, and Google Ngram Viewer indicates that “onomatopeia” is far less common as a spelling than “onomatopoeia” even if we restrict the search to American English.