In connected speech, this may well happen- the sound may either be very small or actually lost.

Student or Learner
Hi All,
I am curious to know that sometimes I hear native speakers say "Diamond" without the sound of "a", like "Dimond", especially in the songs, I always can't hear the sound of "a", please advise. Thanks a lot!
EngFan
In connected speech, this may well happen- the sound may either be very small or actually lost.
Here are some examples you can listen to diamond - Definition from Longman English Dictionary Online.
And the final /d/ may also shrink or vanish.
The 'a' can be lost in 'diary' too, and perhaps other two-syllable words where the stress is on the first syllable.
On the other hand, the 'a' is rarely lost when the stress is elsewhere, as in 'diapositive, diarrhea' etc.
Check these out:
John Wells’s phonetic blog: diamond
PhonetiBlog
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