For those who are not following, here is a graphic illustration: Ashkenazim would pronounce the word 'luck' and 'lock' as we are accustome, but Sepharadim would pronounce them both as 'lock,' were these two words Hebrew.
These two distinct pronunciations is paralleled in the two ways that Syriac is spoken by Christian Aramaic-speakers; a point noticed by Samuel David Luzzatto, which he took as evidence in favor of the correctness (in the sense of conforming to Tiberian Hebrew) of the Ashkenazic qomatz, at a time when Ashkenazic Hebrew was viewed with disfavor (early 19th century, but that view never seems to have properly faded). The difference: there are those Syriac Christians who say 'Abun di-bishmayo' and those who say 'Abun di-bishmayah,' in The Lord's Prayer (both mean 'Our Father who art in Heaven').
An interesting fact in the history of Ashkenazic Hebrew is that, apparently, Ashkenazim (Franco-German Jews) did not always distinguish between these two vowels, like their Spanish, North African and Middle Eastern counterparts. This is attested in early Ashkenazic manuscripts in which confusion between these two vowel's graphic symbols (ָ and ַ) are found in great abundance, exactly as in Sepharadic manuscripts. Although I don't have a handy example of a genuine Ashkenazic manuscript, a picture is worth a thousand words, and I reproduce an image which adorned my post How a 12th century English gentile vocalized Hebrew:
But manuscripts are only part of the evidence.
A juicy piece of evidence for this can also be found in Rashi's (1040-1105) comment on TB Berakhos (or should I say Berakhoth, in homage to early Ashkenazis?) 47A. There the Gemara discusses amens, and says ...אין עונין לא אמן חטופה , "the Amen uttered in response should be neither hurried..." To this Rashi gives the interpretation of what an hurried Amen is: שקורין את האלף בחתף ולא בפתח ואמר אמן והוא שצריך לומר אמן, "He reads the 'aleph with a sheva (אְמֵן, [short first sylabble] E'mein) and not with a patah (Ahmein)" (presumably in Rashi's text the two אמןs were vocalized, but in the standard printed editions Rashi is never pointed).
Thus, we see that Rashi said a word written אָמֵן as if it were written אַמֵן, just like the Sepharadim. Rashi said ahmein, not ohmein. It should be noted that in Artscroll's Schottenstein Talmud the explanatory notes say that Rashi terms the qomatz godol a patah and gives several references; but it does not explain why. Patah itself means that the mouth is to be opened wide; that is how the ah sound results.
Rabbi Dr HJ Zimmels, in his classic Ashkenazim and Sephardim: Their Relations, Differences, and Problems As Reflected in the Rabbinical Responsa discusses their respective pronunciations and adduces other proofs, among them the fact that Rabbenu Asher (13th century), who moved from Ashkenaz to Sepharad, becoming one of the foremost leaders in Spain, discusses the differences between the Hebrew scripts used by the respective communities but doesn't seem to notice any difference in pronunciation at all.
In any event, following Rashi, over the last hundreds of years Ashkenazic Jews pronounced, and still pronounce, amen as something sort of like 'ohmein.' But evidently at this early period in Ashkenazic history the qomatz was not yet pronounced distinctly, and pronounced ah, like the Sepharadim.
The question is when and why and how did this change? Did this come about due to a happy accident, normal sound shifting? That seems unlikely if these two vowels were pronounced identically. How could normal changes come about conforming correctly to the graphic differences in the written texts? Rather it seems like a deliberate effort to conform to the Tiberian pronunciation. Could this have really happened? I don't think there is any evidence that it did. But perhaps gradually it was noticed that Hebrew was spoken in a 'mistaken' fashion, meaning that it did not conform with the graphic symbols, the signs devised by the Tiberian Masoretes. Like other, periodic efforts to improve spoken Hebrew, perhaps somehow this change occurred. Maybe it first occurred among exacting Torah readers. Maybe some pedantic melamdim (children's teachers) taught it to their 3 and 4 year old charges. In any case, it seems irrefutable that at some point in the past 900 years (and perhaps in the past 700 or even more recently) Ashkenazim begun pronouncing the qomatz properly, in the sense that it is distinct from the patah, and in the sense that Tiberian Hebrew is proper Hebrew.
However one lingering doubt remains in my mind: it is generally believed that Ashkenazic Jewry had roots in Eretz Yisrael at a later date than Mizrahi and Sephardi Jewry, whose roots seem to be in Bavel. This is reflected in various Ashkenazic customs and liturgies which stem or seem to stem from an Eretz Yisrael origin. Historically and politically it makes some sense as well, since it is generally presumed that the core Ashkenazic community emigrated from Italy and other areas of Europe which were Christian and thus this Jewry was under the influence of Eretz Yisrael, also under Christian rule, rather than Bavel, then under Islamic rule. If so, it would seem natural that Ashkenazic Hebrew would be more greatly influence by Tiberian Hebrew at the outset. צריך עיון.
( By the way, I recently had my two year Bloggiversary.)
 Credit where credit is due. I came across this reference in a Mail-Jewish post by Mechy Frankel.