There are many sounds that an average ventriloquist makes although they would probably stay away from some of the letters that are hard to pronounce to make their act smooth-sailing and easier to do. They would usually substitute letters that are hard to pronounce with some other sounds.
For example, v may be pronounced more like a strong ‘ee’ sound. There are some more seasoned ventriloquists that can sometimes get away with the letter B but the others are harder to do. Ventriloquists already know the type of mouth and jaw position that they should do in order to make it seem like they are not speaking but they have enough space for their tongue to form the words.