Mark of the Fool Audiobook 4

Soooo I've been listening to MoTF and just got to book 4 and I noticed something. For the first 3 books Travis is almost always doing the little Onomatopoeias, like *booooom* *Whoooom* *Scrrrrrrrp*, but as soon as I start up book 4, it's removed completely. I can read in the text these onomatopoeias are still present, it's just that travis doesn't vocalize them anymore. I'm only on chapter 5 at this point, so maybe they come back, but why did he stop in the first place?