Ok so this is a weird one but it does address the ability to be fluent when you're alone and can feel your throat vibrations. I find that I speak better when wearing earplugs. There can be music playing in them or not, but the fluency is better. On the phone, I often keep one headphone in with music playing and for some reason, I'm more fluent. There may be some science backing this or not (or it's a weird psychological thing), but either way, I see improvement for myself.