Talk to a Dark Elf named Ungrien and choose the "Tell Me about Maven Black-Briar" option. You should get a persuade option and if you pass it, the option doesn't disappear and you can keep choosing it, allowing for maxing your speech..
He is in Riften, and can be found at the bunkhouse late night.
I found this in another forum. I am reposting to share, and that i have confirmed it.