I know that this is almost a year later, but I just wanted to verify that yes, that IS Peter Boyle's real voice, used to portray the civilized eloquence of the 'Monster' after the 'brain transference'. This is my favorite Mel Brooks/Gene Wilder movie. They were gifted with the use of the original equipment from the classic "Lugosi" version. After reading the synopsis of the film on wikipedia, I much prefer Young Frankenstein's comedic spin on things. I've also read the original Mary Shelley book and my daughter is currently studying it in her Honors English Lit class. They are watching Kenneth Brannaugh's version which is the closest to the book yet. I preferred "Frankenstein: The True Story" with James Mason and Michael Sarrazin, but I've not seen Brannaugh's version yet.
Just a small correction to "barboid": there is no "classic Lugosi version" of Frankenstein (although Lugosi eventually played the monster in the 4th sequel.) The equipment you mention is from the original 1931 Boris Karloff film.