As a former teacher of the deaf in the 60s, we had to learn how each sound is produced in the mouth whether with a wide tongue, a narrow tongue, the tongue on the alveolar ridge, air through the mouth, nose, etc. ASL hadn't been analyzed and respected as a legitimate language as yet back then and was used mainly in residential schools for the deaf. The philosophy in the public schools throughout the US was that a person needed to speak and hear to function in the hearing world; thus, teaching a deaf child to speak and lipread was the main focus in the training of teachers of the deaf in those days. For our classes at the U, each student majoring in Deaf Ed. had to go to the dentist and get a plate made for the roof of our mouth. We then had to put tooth powder (there was such a thing) on the plate, put it in our mouth, make a sound, and then draw on a diagram where the tongue hit and add a lengthly description of how each sound was made.
Re: ventriloquism, I imagine, it is possible to train your articulators, lips, teeth, tongue, uvula, etc. to produce an approximation of a sound a different way than most people produce that sound. The reason we, as English speakers can't roll the Spanish "r" is because our tongues aren't used to it.
Also, in "running" speech, it's the inflection that helps our minds fill in the sounds that might not be articulated exactly "right on". The example our Phonetics professor game was the sentence. "Did you eat yet?" This turns out in normal speech to actually be: "Jeet yet?" and the answer might be: "No, ju?"
However, training your articulators to produce sounds without "articulating" is long and arduous work. Seeing people do it in person, I imagine, is much more rewarding than watching it on TV. Fascinating ability - speech ! Fascinating topic and question