Our voice is a huge part of what makes us human. Our voices, our thumbs, the ability to walk upright -- these are the primary physical attributes that separate us from most of the animals around us.
Not being allowed to speak in a power exchange situation can be very dehumanizing. And frustrating. In a D/s sense, it's huge.
My husband and I don't play around with Speech Restriction very often, but when we do I'm usually allowed to make sounds as long as there are no intelligible words. Occasionally the restrictions include no sounds at all, though I'm rarely able to pull that off.
I know a couple who have speech restrictions within scene as the general rule, not as the exception. I believe she goes into subspace faster, and I believe she reaches that super-deep subspace a lot more often than I do.
My husband likes to hear me talk, beg, plead, tell him when it hurts, ask permission, etc. And to be honest, I much prefer being able to talk.
There is something to be said for restricting speech for a period of time, though. The mindset of not being allowed to communicate anything verbally -- it is objectifying and dehumanizing in a way that I wasn't really prepared for the first time. Even now, sometimes the intensity of it hits me like a brick.
When you're gagged you can usually still get your point across, get enough of the syllables out to be understood. But when you've specifically been told you can't speak, can't communicate in any way, even when not physically gagged...
How do you feel when a Top takes speech off the table for the submissive? Is it too much? Or is it a big turn on for you?