I saw a sentence:
Why is ‘は’ instead of 'が' or 'を' used here? I think the full sentence should be like the following.
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First of all, あなた is never used like the impersonal you in English. It always refers to a concrete person (often someone you don't know the name of). Even when it's used in an advertisement, it's understood as referring to the listener as an imaginary yet concrete potential customer. When a sentence begins with あなたは, it's specifically talking about that person.
Your second sentence, therefore, will be understood as asking about the ability of the person you are talking to, but I don't think that's what you mean here. (It's not a "full" version of the first sentence.) You probably want to ask about the availability of this for online listening. Then marking これ as a topic is a natural thing to do. You can still say これをオンラインで聞けますか (without あなたは) without changing the meaning much. The doer of the listening will most likely be understand as you as the speaker if said in the same context in which これはオンラインで聞けますか is natural.
To me, the difference is like the below.