I used to know a guy who worked at a local radio station back in the seventies. He once told us that the radio stations would speed the songs up by about 10% so that they could stick in an extra commercial every ten songs or so.
He said that the surprising thing was that it made many of the songs sound better even though most people couldn't actually tell that the songs were slightly speeded up.
I've listened to many of my old favorites since the speedup practice was stopped and I think he's right.
I don't know if you've ever heard of this or not, but thought I'd mention it.
:)