Ronny, Thanks for the reply, typically having spent 30 mins working around it, it now won't do it! first noticed it last week and managed to work around it by duplicating audio underneath. Done a restart and now seems fine, I just really wanted to know if it was a bug someone had seen before, guess it's just me and I'll have to live with it occasionally!
Thanks for the update. As this is not an issue that has been reported anywhere before, and restarting did the trick, we can classify this as an unexpected error. If you still get this issue in the future, you might consider refreshing your FCP X preferences.