Without being able to pinpoint the actual problem or see mIRC's source there's simply no way we can discount anything with what little information there is.
That's why I added the url to the MCI DLL, if they have the same problem when they try to play the file with the dll, then it has nothing to do with mIRC as in my situation. I guess though it would be kind of difficult for a normal user to figure out how to use the dll.
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Also, they need to make sure the MCI drivers are installed on their system, though they should be as they're installed with XP/Vista by default.
Sound, video and game controllers > Media Control Devices
