Simply because of how 'controversial' the mumbling is, I agree that some sort of voice volume bar would be nice.
However, I disagree that the mumbles in Yooka-Laylee are random. Trowzer, for example, has an obvious 'wise guy' wheeler-dealer voice -- and he does hiss, not in his voice, but when you stand next to his character model and he sticks out his tongue. You are standing next to him 99% of the time you speak to him, so you do get hissing in with his voice effect.
If Capital B buzzed, I would probably think it was ridiculous. His voice resembles a heartless corporate boss, which makes sense to me.
The one voice that I don't particularly like is Clara's, the skeleton girl, because it is a hollow xylophone noise, which is ironically what you would expect a skeleton to sound like. I actually prefer it when Playtonic uses voices that represent the characterisation, as with Trowzer and Capital B, rather than their species or object.