FOX swapped the music used in the shows for cheaper tunes that could be licensed in perpetuity, and when the original song couldn’t be licensed, the scene was cut.
Justin Levine went through this exercise the first time, when the show was sold into syndication. Working at MTM, Levine had to replace the popular rock songs with generic upbeat Muzak that could be used with a one-time, flat licensing fee. So scenes dependent on a musical joke were ruined, like Les Nessman trying on a toupee to the soundtrack of Foreigner’s Hot Blooded. The DVD release made even more cuts.
In 2005, Wired predicted the show would never make it to DVD.
Jaime Weinman explains more about the copyright issues and the costs involved.
So, what to be learned from this? Don’t set a sitcom in a radio station.