I'm probably not the only one to notice this, but in the last few seasons, the voice acting has overall taken quite a significant decline. Most noticably, the voices sound quite a bit different. I guess after 20 seasons, the actors age and consequently so do the voices, but it's been quite drastic between the most recent seasons. The delivery of lines also seems to have changed for the worse too.
The reason I brought up this topic is because I was listening to the season 9 commentary, and the actors (namely Dan and Nancy) seem to perfectly nail their classic-era voices. So what is it that affects the quality of the voice acting in such a negative way?