What is the impact of the voices of Victor Frankenstein and the monster on the readers?
The impact that the voices of Victor and his monster have on the readers can be vastly different. Depending on which character one relates to, and empathizes with the most, determines the impact of each on the reader.
One could "listen" to the tale of the monster and feel great empathy for him. If a reader has felt abandoned by a parent then the reader will relate to the monster much more than with Victor. In fact, the reader may find Victor despicable for abandoning the monster.
Others may see Victor as the more sympathetic character. Given his great loss at the hands of the monster, some may find the monster the deplorable one and Victor the one needing sympathy.
Therefore, it completely depends on the individual reader with how the voice of both Victor and the monster impact them.