Rumbling from the elephants is essentially their way of communication. In both videos, you can hear Mali communicating back, probably communicating that she's coming and on her way back. Once they're all together, you can hear them still trumpeting and rumbling - the keeper describes it well, they're effectively just ensuring that one another is safe and everyone's alright.
I too have rarely heard elephants rumbling in person but it is common when cows are in with a bull or interacting with one - I recall Kulab and Luk Chai communicating in that manner through the gate a few days after his arrival from Dubbo.
I’m not referring to rumbling/trumpeting, which like you say is a common form of communication, I’m referring to the roaring noise, which is this instance is clearly being used by Roi-Yim to express distress/displeasure. The herd recognises his distress and responds to him, which is good to see.
A similar example is after the birth of Pathi Harn, where he rolls under the gates, Pak Boon finds Porntip in a state of stress (as she can’t reach her calf). She lets out a loud roar, which brings an immediate response from Thong Dee, Tang Mo and Luk Chai, who were asleep in the barn at the time.