Another part of the reason for the overdubbing of voices is that for some of the episodes, the people who changed the music didn't have access to music-free tapes of the show, just the final mix with the music -- so when they changed the music, they had to add new voices and sound effects too. Very clumsy all around, of course. The music changes were made in the late '90s, when MTM had almost completely collapsed; I think they were trying to rush WKRP back into syndication to make some quick money, and they didn't have anyone who could do a proper job of music licensing, so they just changed everything as quickly and cheaply as they could. Fox bought the MTM catalogue a couple of years later, but while there was some talk of them remastering WKRP, it hasn't happened, due to the music issues.
There should be an agency that specializes in tracking down rights holders and negotiating for synch rights on behalf of home video companies. I would guess a lot of the problem is that companies like Fox HV don't want to tie up several employees for several weeks doing this.
Harry Fox used to do that, but stopped 3 years ago. I don't know why, and I don't know why nobody else has filled the void.