All the grammar tips say that "could" can't be used when it's meaning a single past event. They say all the things like hypothesis mood or general ability or whatnot.
But, then again, at the end of their explanation, they would usually put an exception of this rule for certain verbs or adverbs. Why is it so? If you say like
I could hear his voice yesterday night.
,then would the verb 'hear' make the conditional mood wear off? If so, how come?