In Homer's earliest days i.e the original Tracy Ullman shorts from the 1980's and the first season of the series Homer's voice sounds completely different from the present day version, being noticeably slower in pitch and lacking the feel of modern-day Homer. The explanation for that is that when the character first started, voice actor Dan Castellaneta originally based Homer's voice on a Walter Matthau impersonation which was slower sounding and more drawn out. A perfect example of this would be the line "Let's all get frosty chocolate milkshakes!" said often by Homer in the original shorts and very early episodes. Castellaneta also claimed that this version of the voice was too hard on his throat and too difficult to sustain for a prolonged length of time, thus the newer version of Homer's voice was developed based on the voice actor's own father, and is what we are familiar with today. No official explanation was ever offered by the writers of the show for Homer's mysterious voice change, so it just became yet another unexplained sitcom anomaly, like the disappearing cat on The Brady Bunch or Richie Cunningham's disappearing older brother on Happy Days.