In a way, these passages [of Whitman’s] present a challenge to the modern academic terminology of “the speaker.” In this critical tradition, students may discuss the words not of John Donne but of “Donne’s speaker,” and even (though this sounds more peculiar) not the words of Emily Dickinson but of “Dickinson’s speaker.”

Useful though the notion of “the speaker” may be sometimes, it is challenged by certain poems.

Ben Jonson’s “On My First Son”: Does “the speaker” matter? – Slate Magazine.