An inner voice or inner monologue as you describe isn't necessarily a good thing.. The idea that some people can't relate, is pretty weird.. but mindfulness is indeed about getting that voice to shut up so you can experience observation in its purest form..
There are a lot of interesting reads on Thought and Language (which includes, thought without language)
When you take entheogens for example, you can go either way.. your inner voice can seem out of control (or in control of you) OR all your preconceptions including your relationship with language can temporally collapse and you see everything as though for the first time.
But I believe there's people out there that actually don't experience.. at all. .they just aren't experiencing anything.. the lights are on, but nobody's home.. Much scarier to me than people who don't have much of a monologue going