How to pronounce องุ่น
There are several words that start with อ fused together with another consonant letter. So how do you pronounce a silent letter with another consonant letter?
Well, it's the same as any two 'unpronouncable' fused letters. You must open your mouth wide to jump from the first sound to the second, i.e. by making an ะ sound.
So องุ่น is pronounced อะ-งุน, just like is สบาย pronounced สะ-บาย or ขนม is pronounced ขะ-นม.
Obviously, there are a few two-letter words that start with อ and have no vowel (in which case it's the invisible or default 'o' vowel), like อก (chest/breast), อม (to suck), อบ (bake), อด (refrain from) and องค์ (organ/bodypart).
And don't forget the four words where อ changes the ladyboy (ย) back into a boy for tone purposes:
Note: For tone purposes, it's the first consonant that determines the sex of the word (or compound syllable in this case). But also note that this only applies if the second consonant is a ladyboy (which it is, most of the time, anyway)...
So องุ่น is a dead boy (sad).
ขนม is a singing girl (question)
But in สบาย, the second consonant in the fused pair is a boy - so we don't give precedence to ส and so the sex for the whole thing is a boy: บ (i.e. singing boy = no tone).