I had this same problem a few years ago and went down the rabbit hole. I was looking for the US Robotics X2 and K56Flex handshake sounds and eventually found them.
Turns out I've been trying to track down the USRobotics Sportster voice modem sound from your first link. Even better as I can't find my 56k USRobotics, only the 33.6k one, so probably wouldn't have ever found that sound with hardware either.
Thanks; that's been lodged in my head for a couple of decades.
V.90/92: https://goughlui.com/2016/05/03/project-the-definitive-colle...
X2: https://goughlui.com/2018/06/10/project-record-a-us-robotics...
K56Flex: https://goughlui.com/2018/06/10/project-record-a-k56flex-dia...