Mr. Hankey's Christmas Classics - S3-E15
Corrected entry: During the dreidel song Kyle's mother comes in, but you can barely hear her voice. This is because the woman that played all of the female voices committed suicide in the middle of the 3rd season, so they had to find another person and they turned down the sound of her voice so no one would hear any differences between the real voice and the voice they used in this episode.
Correction: Not quite. It is true that Mary Kay Bergman killed herself in the middle of season three, but it actually is her voice in this episode. The songs from this episode were from an album the creators put out of the same title (Mr. Hankey's Christmas Classics.) The album was recorded and released long before this episode aired. They simply animated an episode to the album because they were stuck with no female voices after the only one was dead and had no other ideas or time to think of any. The reason Kyle's mother sounds like that is due to the mono sound systems on most TVs. Since albums are meant to be heard on CD players, stereo versions of the songs are usually created. If a stereo song is played on a mono device (such as a TV,) some sounds may be muffled or completely blocked out (if you don't believe me, play Sgt. Pepper's Lonely Heart's Club Band on a DVD player with no surround sound system or stereo.) Matt and Trey probably had no time to worry about stereo/mono conversions and simply aired it as it was. When I watched my season 3 DVD on my portable DVD player (which has stereo,) her voice was at normal volume.