If the Buddha was to say something directly to me, it would be easy to accept the validity of sabda-pramana.
But on what grounds? On grounds of your faith that he is a Buddha? You cannot directly perceive his Buddhahood but just believe in it.
But I was not there to hear him speak. What we have are texts, languages, interpretation: can we talk about knowledge before we talk about hermeneutics?
Really ... what is the difference between seeing a person and having faith in what he says and reading words and having faith that those are true? What makes the alleged difference a valid one? I tell you: Your will.
Then there is the possibility of hearing the discourse of someone contemporary who claims yogic insight: but how do I know that they have yogic insight?
Exactly, how can you know ... how can you know anthing at all?
I can assume. I can be open to the possibility. But can I really know
? I do not have access to their mind. I do not have the refined abilities to discern. So how can I really know this?
The only way I could really
know would be if I
also had yogic insight: the yogic insight which can perceive when others have yogic insight! Without such a thing, there can be receptiveness, there can be openness, there can be learning, but I do not think that there can be adequate grounds for knowledge.
I assume that you are actually mixing up categories. Perhaps you think that sabda-pramana must be pratyakşa. But actually it is a form of anumāna and as such the validity it may have is depending on the conventional logic you are following.
But you also have to follow the conventional belief that someone is a "Buddha" and have faith in the conventional meaning of "Buddha" in order to believe in his words if you hear them directly.