You could always create an alias of /splay that echoes $1-. Off the top of my head I can't think of any other way to stop a sound from playing so if it is a script it's almost definitely going to be use of the /splay command that's the issue.
Of course if the script happened to be using /!splay then an alias won't work, but try you could always search your scripts for "!splay" - I doubt there'd be many lines that are likely to match it so it should be easy to find if that's the problem.
If both of those things fail you could always turn remotes off and see if it starts working.
And if that doesn't work try re-installing mIRC over the top and try again. It's just possible it's some random corruption of God-knows-what causing the problem. It may be the computing equivalent of kicking the damn thing but you might be suprised how often it works when all else fails.
