The whole album, so I've been told, had been re-recorded for a stereo release with changed lyrics for some songs (This is the version you'll hear in this video, although it's from a mono pressing) The wikipedia page for the album has some more info: http://en.wikipedia.org/wiki/Songs_by_Tom_Lehrer
Sorry about the small cut-offs at the starts of each song. I don't know why WLMM does that.