The words speaker and hearer are countable nouns according to the Cambridge Dictionary. And a countable noun should either used plural or be with an article.
But I see many use these words without any article in the singular form. For example see:
It is very plausible that this transfer of thought contents from speaker to hearer is a necessary condition for successful communication. Source
So, my question is, can a countable noun become uncountable just by our usage?