(They are talking on the phone)

A: You have turned up the radio.
As it's too noisy, I can't hear you.
B: Sorry, I will turn down the radio.
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In this short dialogue, doesn't "it" in "it's too noisy" refer to "the radio"?

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The dog is barking now. As it is too noisy, I can't hear you.
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In this case, does 'it' refer to 'the dog'? Then 'it' is a personal pronoun refering to 'the dog.' Am I right?

Yes. Both of your answers are right.

Yes, you are correct in both cases.

In the first dialogue, when person A says "As it's too noisy, I can't hear you," the pronoun "it" is referring to "the radio." So person A is saying that the radio is loud and that's why they are having difficulty hearing person B.

Similarly, in the second sentence, when someone says "As it is too noisy, I can't hear you," the pronoun "it" is referring to "the dog." This means that the person is saying the reason they can't hear the other person is because the dog's barking is making it too noisy.

Pronouns like "it" are used to refer back to a noun or noun phrase already mentioned or understood. By using pronouns, we avoid repeating the same noun multiple times and make our sentences more concise.