"ag + verbal noun" is the equivalent of the gerund in English - "hearing". "a + verbal noun" is usually used to translate the infinitive "to hear" (that's a very simplified explanation).
I prefer to translate "ní féidir liom" as "I'm not able" ("to hear you") or even "it's not possible for me" ("to hear you") to preserve this structure - "I can't", even though it has the same meaning in this sentence, drops the "to".
Read those notes again. They are explaining the use of the verbal noun in the present progressive (to be + ...ing). (In fairness, the notes don't make that clear).
"Ní féidir liom sibh a chloisteáil" - "I can't hear you".
"Níl me do bhur gcloisteáil" - "I'm not hearing you".
You're approaching this the wrong way round.
If you think of "ag léamh leabhair" as "at the reading of a book", you can see the genitive construction. You can also see how you could recast it with the possessive - "at a book's reading". When the object of the verbal noun is a pronoun, we stick with that possessive form - "reading me" - "do mo léamh" (at my reading), "reading you" - "do do léamh", etc. (the transition between "do" and "ag" and "á" doesn't change this - they are all basically the same thing).
So it's not that you can't say "ní maith liom a bheith ag cloisteáil sibh, because there is no genitive form of sibh?", it's simply that you don't, because the possessive pronouns don't require the genitive, whereas you do use the genitive to mark possession in other cases. (obviously, you don't possess the verbal noun, but you can see how it fits into the same grammatical pattern).
Thank you again for your explanation. Unfortunately, I could not answer or see it earlier because I do not get any notifications anymore. I have already reported it and hopefully it will work again some time in the future. ;-) I got an Irish grammar book two days ago and now I can look out for more answers.. Go raibh míle maith agat