Just wanted to report that I am seeing the same behavior (same strange high-pitched voice) on Ventura. It appears to be a bug.
However, after some testing this seems to only occur with the MacOS system voices. I have a Cereproc voice installed, and if I switch the setting under “Spoken Content” to use this voice instead of a system voice, then it is used in MarginNote instead of the high-pitched one.
Anyone considering this as a workaround: The “Adam” and “Nathan” voices from Cereproc are quite good, although I now actually prefer Jamie Premium (which is a MacOS system voice and available for free).