Users with the problem can't play any MP3 files with mIRC. Is not a file encoding problem, because the users have a lot of MP3 from different sources, with different encoding, with different bitrates, etc.

But the fact is that these MP3 works with others players.

I don't know what is wrong. mIRC don't play MP3, but I think it reads the file header to pass to the system to play, and I think the problem is here, in the mIRC or in the system (or both).