Why is cholam sometimes pronounced like segol (Chabad, Yom Kippur)?
I learned what I understand to be a S'fardi or maybe Israeli pronunciation of Hebrew. I'm aware of the differences between this and the usual Ashkenazi pronunciation. My question is about variations I've heard in Chabad. Having spent Yom Kippur with Chabad this year, I heard the differences enough to convince myself that they weren't just passing things that maybe I misheard.
(I am unsure of whether I should be asking this question here or on Languages & Linguistics; since the answer might be affected by the religious context I'm trying it here.)
I noticed, in particular, that sometimes a cholam would be pronounced like a segol, but not always. From the same speaker, for יוֹם I heard both -- sometimes with a cholam as I expected, and sometimes with a segol. I also heard this difference in עוֹלָמִים. I did not remember specific contexts by the end of the day when I could take notes, sorry.
Are there places in the Yom Kippur machzor where there is some sort of k'ria/k'tiv distinction going on, and I would hear these same variations in other traditional basically-Ashkenazi settings? Is there some contextual clue that I haven't noticed? Is this something specific to Chabad or, perhaps, my local Chabad community?
There were other pronunciations that were different from what I expected. (I've been told there's a Yiddish influence.) This is the one I remembered enough specifics of after to be able to ask. It's possible that this is one detail of a larger pattern, and if so I'd welcome the general explanation.
I sometimes had trouble following the service even though I had the text in front of me. I'd like to be able to do better next time. I ask this question with that goal, recognizing that this is my deficiency, and I hope I am not sounding in any way critical when I'm just trying to understand something unfamiliar so I can learn it better.