Question: Is it natural and correct to say *talk via audio* meaning to actually talk to someone over the phone rather than text? For example:

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> Do you prefer to talk **via** text or **audio**?
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If it sounds off, would it be more natural to say *talk via phone call*?

Answer: 
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> A friend of mine
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=

My friend

You are, I suspect, misunderstanding the distinction between "a/an" and "the." The indefinite article implies that a single unspecified member of a set is being referenced; the definite article plus a singular noun implies that a singular but previously specified member of the set is being referenced. Neither usage implies anything about the size of the set (except in mathematical terms it is not empty).

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> I have a broken finger
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does not imply that I have only one finger. In all likelihood, I have ten. What it says is that one of my fingers is broken, but it is silent on which one.

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> My finger is broken
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means the same thing. "My" here has a similar meaning to the indefinite article.

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> The forefinger on my left hand is broken.
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I am now specifying which finger is broken so the definite article is appropriate.

Just as good is

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> My forefinger on my left hand is broken.
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In short, possessive pronouns like "my" can replace definite or indefinite articles, and definite and indefinite articles relate to specificity of reference
The sources you’re referring to are either misunderstood or flat-out wrong.

“My friend” simply means that the friend I’m talking about is (one of) mine, rather than someone else’s friend. It says nothing about how many friends I have.

If you need to clarify that you have exactly one friend, you would say “my only friend”.

There are certain things that people are presumed to have only one of, like hearts. If a person had a transplant, they might refer to their old/new heart, but without a qualifier, we presume they mean their current one.