Files are MP3. Affected users have thousands of MP3s and they can't play none of them with mIRC. But it works with Winamp, Windows Media and others players.
For example, one of these MP3 files have this Mpeg info:
Size: 7712976 bytes
Header found at: 2048 bytes
Length: 482 seconds
MPEG-1 layer 3
128kbit, approx. 18491 frames
44100Hz Joint Stereo
CRCs: No, Copyrighted: Yes
Original: Yes, Emphasis: None
Its a MP3. It works correctly in my mIRC but not in the affected users mIRCs.
That users have MP3 codecs in their Windows. Some have installed fraunhofer codes, and others are using the XP native codecs. But they cant play MP3 using mIRC.
I'm not telling that after renaming to .wma the file is converted to a wma, please ... But the fact is that after the .mp3 is renamed to .wma or to .wav, mIRC plays the file.
I think isn't a codec problem. I think the problem is the way of mIRC recognize the format and call the codec after. Can be a Windows media problem, I don't know.