Yes, /say works in 6.31. You'd have to show the script that isn't working to get much in the way of help. That said, one of the most common causes of a script no longer working when upgrading to 6.3 or 6.31 is not having the right number of ()'s on IF statements and similar commands.

Previously, something like:

Code:
  if ((this == that) { do this }


... would work. Noting the missing ). That kind of error will no longer work in 6.3 and 6.31. That's just a common cause of scripts not working when they used to. It may not be your problem, but it might give you an idea of what to look for. In the end, a script no longer working is going to most likely be some error/bug in your script that was ignore in older versions of mIRC.