This was discussed awhile back as well, Brax. Before continuing to read this, keep in mind that I'm writing this from memory and it may not be entirely correct. I'll leave it to argv0 or others to correct any errors.
Basically, mIRC uses the extension to choose what codec to play the sound file with. Unlike regular media players that will try the codec that matches the extension first, then proceed to try others (or just read the file's header information if there is any), mIRC only uses the extension. I believe this has been suggested before as a feature/improvement to have mIRC work like WMP and other media players. For now, you really need the right extension or you'll probably have trouble playing the file.