Hi Guys,
I just spotted your thread after an hour messing about copying an audio clip from one track to another - and even though the clip was fine playing on the original track, it just wouldn't play on the track I pasted it to.
After reading the above, I selected the vocal clip on the track (the one I had moved and was still in it's new situ), then clicked on EDIT followed by SELECT MUTED REGIONS and it cured it.
It must cancel a mute that's not visible and apparent to the user on the arrange page.
Apple should correct that with a different coloured bar attached to it or something.
Anyhow, happy days!
Jules