The problem you have is because a codec you have installed. Probably an AC3 codec.
The AC3 codec that comes with Nemo codecs pack can cause this problem.
It is infact a problem with Windows/the codec and not with mIRC.
Altough winamp and Windows Media Player can play the mp3 files fine, if you will try to play those files with Mplayer2 (the mplayer2.exe is located at the same directory of WMP), you will hear the high pitch sound.
As much as I know, the only thing that can be done is uninstalling the codec that caused the problem.
