Can someone please tell me or guide me to a good explaination about why vowels sounds change in a word. For example, “Costco,” the first o sounds like ah and the second o sounds like…well…o.
Thank you. 
Anal is similar.
Banal is different.
Sorry, I’m being a dick. Two syllable nouns generally go down on the second syllable, two syllable verbs generally go up. Proper nouns like Costco can have their own rules.
Again, I apologise. You’re asking about vowel sounds. I think this is about accent. Costco for me is an o followed by an o sound. The first o would only be an ah with North American pronunciation. Cahstco wouldn’t be something I would say.
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Costco is a name, and names can have their own rules. I think there is no rule to change sound if word has two vowels.
Co = short version of word “company”. Always pronounced as “ko”
Cost = normal word pronounced closer to “ka”
Costco = Ka-st-ko
Another example is Tiffany & Co (the fashion company)
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The syllable structure is relevant. Syllables with an individual vowel letter ending in a consonant normally have a short vowel sound, as in cat best strip on flug. Syllables ending in o usually have a long o sound. Go no slo mo
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Costco isn’t a real word. Use a cucumber.
It’s probably wise to consider IPA as the basis for pronunciation, not English letters. The letter ‘o’ can have a variety of sounds: from ‘ah’, ‘ooo’ (long) to even ‘u’ (bus) depending on the word, the surrounding consonants/vowels, convention and even its origins.
The rules are complex and at times contradictory. IPA gets around this.
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I’m unsure exactly what you’re asking.
If you mean why do the five vowels have more than one sound, then the explanations can be found in the history of English.
To remember how they sound differently in different contexts, then there are some patterns that can be remembered: the same combinations of letters consistently pop up again and again making the same sounds. Except when they don’t. So you also have to remember the different ones.
For example, the interesting shift in pronunciation of the second o as you move from photo to photograph and then photographer is a pattern that repeats in similar words, along with the change of the first o from photograph to photographer. But you have to remember that photojournalism is using just the photo shortening in a new compound word and so carries its pronunciation. And photon belongs to different class of modern words, so matches their pronunciation. If you look at other words with two o sounds, like colon, the sounds change but in different ways when you lengthen them: colonoscopy, but then colonoscopic.
If none of the above was what wanted, then if you clarify your question I could probably write something more helpful, because questions that I cannot answer often prove an interesting diversion.
Because of the American accent, they tend to pronounce o as ah
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