I think perhaps all the other music featured that Season in both the US Region 1 and UK Region 2 except those particular episodes had their music changed due to music licensing. Wouldn't this qualify as preservation of the original broadcasted audio and not a custom audio track?

Season 1 US retained original music, Seasons 2 and 3 UK retained original music, Season 4 replaced it with generic music for those episodes and Season 5 both US and UK has original music. I just got Season One as a late birthday present from me Momma when we where hanging out yesterday. Seasons Two and Three of course will be purchased from Amazon UK, Four and Five will be purchased via Amazon Marketplace.

As I was saying, someone could grab the original broadcast music, grab the particular episodes and replace said music? This I hope wouldn't fall under the "Regarding fan DVDs with alternative audio tracks" since it'd be a preservation of the originally broadcasted music for these episodes on Season Four? I'm quite the fan of this show.