A question about The Bell Jar by Sylvia Plath.?

If you've read the book, you know the line "I listened to the old brag of my heart. I am. I am. I am."

I did research and noticed that some people say it's "bray" not "brag"

I guess I have 2 questions, if it IS brag...what would that even mean? Because the word bray fits better.
My other question is, does anyone even know which one is right? Brag or bray?!