That does not sound like English to me, but it does sound like a built in voice for macOS. usually, macOS will use an audible voice like that for some of the following:
Reading text (this is unlikely because since you are not actively telling the system to read text)
The system can announce the time, could this have somehow been activated for the system of an announce each hour?
AppleScript can be used to make the computer transcribe something. Do you have some kind of odd AppleScript running?