Two of the most common questions I get are: "What happened to [song title] on the DVD?" and "What song is that on [episode] -- it's not the one listed on your episode guide."
Here's what happened:
When Northern Exposure first aired on TV, the music played in each episode was almost like another character on the show. It was filled with an ecclectic collection of tunes much like Chris-in-the-Morning would play on KBHR (and often did!). One of the show's creators (Joshua Brand) was very interested in having music featured as it was on the show. He enlisted an associate producer, Martin Bruestle, to help find the perfect tunes out of their own collections, nightclubs, radio station basements and tunes heard everywhere. [Read more about the original music here.]
As is required by ASCAP, the producers had to secure the rights and pay royalties to use these songs on TV (which extended to future syndication, a handful of videos and Laserdiscs).
Flash forward a couple of years and a new video media is introduced: the DVD. Since those original rights did not included DVD (since it didn't exist and they were inclusive of "future media"), the rights had to be re-secured for the music. This is a costly and timely procedure. Several of the songs were either too expensive, too long to get the rights, or just unavailable for various reasons. One example: Procal Harum's "A Whiter Shade of Pale" (at the end of episode 4.4 Heroes) was in legal limbo over who had ownership to the song. [Read more about the lawsuit here.]
Enter "Royalty-free music". The first season of Northern Exposure was released with almost all its original music. As it was the first season, and the show had started as a low-budget production, those rights were easily re-secured. As Universal prepared for the second season, they found they needed to change many of the tunes. Unfortunately they chose rather bad "Casio keyboard/elevator music" and the fans were in an uproar. Northern Exposure was not the only show with these issues - other TV shows to DVD had the same problem. [Read more about the fans' reaction here.]
Universal then bought or built their own "Royalty-free" music library. This is where studio musicians and songwriters create music that does not belong to a specific artist. Music from such a library can be used freely in projects without having to pay royalties for the rights. This also means there is no title (except probably an ID number) and no artist to list.
As seasons three, four, five and six were released, the replacement music improved immensely. So much so, that my in-box frequently had requests for "What is that song??" My answer is apologetic, explaining the replacement process and unfortunately not having an answer for name and artist.
This process has also opened up a couple of other requests: "Are there DVDs available with the complete music?" (Yes, but they are bootlegs - not Universal-released); and "Will there be official DVDs with all the music intact?" (Probably not -- too expensive to produce, and the issues weren't just cost -- some songs are not available for various reasons).
As we move into the next genre of video media, this issue will come up again. Video-on-demand and internet TV sites likely have the same rights as syndicated TV, so should have all the music when that is available. (Note: I'm not aware of Northern Exposure being available in these formats yet, but if either is available in your neck of the woods, let me know!).