In the JP version, starting with Chapter 2, Behemoth occasionally said “む/Mu/Moo” or “むん/Mun/Moon”. Behemoth the biblical beast is described as a giant creature that ate grass like cattle. It was depicted as a bull, elephant, and hippopotamus; essentially a giant herbivore. Suzune and Tsukinami made their version of Behemoth a “Cow-Girl”, she literally Moos like a cow.
I chose to translate her Moos as “Mrrm” and “Mm”. Back in Chapter 2, I felt just writing “Moo” or “Moon” didn’t properly convey her emotions enough, depending on the context I wanted Behemoth to sound annoyed, happy, or sheepish. If her lines were voiced, this wouldn’t be an issue; a lot harder to understand when you’re reading it in EN.
As for the new girl, in JP she does indeed speak with elongated vowels like Bao Huang from One Piece. As Kumo Onee-san & Behemoth are both anomalies, it kind of makes sense for the both of them to not speak entirely like normal humans. Even the goblins have their own unique way of speaking human language.