I had the same problem with a couple of mp3's a couple of years ago, they played fine with WMP (9) and WinAmp (2.91) but not mIRC, so I tried to play them with
DragonZap's MCI DLL (
Media Control Interface) which uses the same method that mIRC uses, and it wouldn't play them either.
~ Edit ~
I forgot to mention that out of the thousands of mp3's I had at the time, only two would not play with mIRC, the rest were fine.
Affected users have thousands of MP3s and they can't play none of them with mIRC.