Now, when a diphthong is placed in an unstressed syllable, it can undergo a process of weakening too. The second vowel can be considerably debilitated, often to the point of being dropped altogether, especially in rapid, everyday speech, but not exclusively. This is particularly true of the diphthong /əʊ/, like in the following examples (in dictionaries this is sometimes signalled with the second element in italics /əʊ/):
obey /əʊˈbeɪ/ → /əˈbeɪ/
paleolithic /ˌpæliəʊˈlɪθɪk/ → /ˌpæliəˈlɪθɪk/
Listen to these two recordings of the word obey.
First, American actor Tom Hanks, who keeps the diphthong and pronounces /əʊˈbeɪ/.
Next -and it seems like a joke, right now, really- British Prime Minister Boris Johnson addressing the nation and admonishing people about the consequences of not keeping the rules during the coronavirus pandemic. Johnson, on the other hand, clearly reduces the diphthong to a schwa /əˈbeɪ/.
You must obey the rules on social distancing and to enforce those rules we will increase the fines to the small minority who break them (Boris Johnson, officialy addressing -and menacing- the nation).
There are also other cases of weakened diphthongs, other than /əʊ/. The contraction they’ll is very frequently pronounced /ðel/ instead of /ðeɪl/.
We’re /wɪə/ is often reduced reduced to /wi/.
My /maɪ/ is sometimes pronounced /ma/.
I know some people might consider this careless pronunciation, but actually it’s how real English is spoken very often, even in formal contexts. Of course, the weakening of diphthongs is not obligatory and it happens only occasionaly, not all the time. In any case, to assuage your possible misgivings, listen to the voices of two well-known and very well-spoken BBC Radio 3 presenters:
Can you hear any diphthong in my? I think the glide /ai/ is so faint that is barely distinguishable.
ǀ maɪ ˈfeɪvrɪtǀ → ǀ ma ˈfeɪvrɪtǀ
And this one is even more clear:
ǀ maɪ ˈmʌm ǀ → ǀ ma ˈmʌm ǀ
Also, as I’ve already explained in a previous article, normally the British diphthong /eə/ is only pronounced as a diphthong when it is stressed or comes before a pause, as you can see in the following example:
In theory, the contractions they’re and the adverb there are both pronounced /ðeə/ (they are homophones), but in this recording you can clearly appreciate how the unstressed, non-final they’re is pronounced just /ðe/ (the reduced or weakend form) whereas there, which is the last element in the clause and it’s stressed, retains its strong form /ðeə/.
Let’s now read what professor Adrian Underhill has to say about the weakening of diphthongs: “Unstressed diphthongs become generally less distinct, often losing their two-vowel glide quality and merging into one composite `greyish´ monophthong”.
I think this recording by Elvis Costello is a good example of Underhill’s “greyish monophthong”. The vowel he pronounces in the unstressed syllable of myself turns into a sort of schwa.
myself /maɪˈself/ → /məˈself/