Subvocalization is and interesting idea, but it's more about how to encoding information into your brain, how about the decoding process? For instance when you write an English essay can you finish it without a speaking voice in your head?

And oh yes, we all think in formulas when doing mathematics. I have problem vocalizing formulas with Greek letters to others but I can quickly write them down. I think it's because Greek letters were never officially taught in China.