During my research, I came across the MSM6322 speech pitch control chip. It can be controlled very easily via a microcontroller. You can change the voice in real time, which is pretty cool. Since the chip is still available from some sources, I decided to use it. The first draft of the schematic looks like the following:

PCB layout:

Populated and tested circuit PCB. Works as desired:

The microphone holder consists of a custom-made part and parts of a cheap clip-on microphone. I soldered new wires to the electret microphone. I also made a sound box for the speaker.