I notice no one talks much about this while having a problem about it. Usually people complain about their non-vocal sentient tulpa just hardly managing to talk if at all but don't know what to do.
So, if your tulpa is trying to talk, or can't really do it, I found a helpful method for getting them to a point where they can start figuring out how to talk.
First, if your tulpa has ever talked, even just once, figure out what their "mindvoice" sounded like. Now, get your tulpa in your wonderland, and tell them what you're going to do. You're going to use their mindvoice to say a sentence, and then the tulpa can either try to say it with you at the same time, or you can say one half and they say the other. It should be a simple sentence such as "the rain in spain stays mainly on the plain". Make sure to tell them what sentence you're going to practice on of course.
If they don't have a mindvoice, you can either wait until they try to talk or just use whatever voice you want them to use, I imagine they would pick it up anyway.
I did this after my tulpa was just barely able to get a single word out, and in just one night we got to a point where she could make whole sentences. While she wasn't too good at talking, we got to a point where it was up to her to practice, so I didn't need to work on it anymore.