I think its not exactly a mIRC bug, because mIRC plays the MP3 correctly in the most systems. But some users have this problem, and I have no idea why.
What you said there clearly shows it isn't a mIRC bug. If it's only on certain systems (and very few systems at that), then it's not going to be a mIRC issue, but an issue with those systems or their settings. As has been pointed out, mIRC doesn't do anything except send the file through Windows to be played.
If it's not working on someone's computer, then either they are using /splay incorrectly, sounds are disabled in mIRC, WMP is not installed (can be an issue depending on the OS version), or the files are incorrectly named and/or corrupted. Beyond that, I have no idea and it would have to be worked out on those systems to find the problem.
I will reiterate that mIRC doesn't actually play the file itself. It sends it through Windows and it is up to Windows to actually play it. If mIRC can't play it, then any other program that sends the file to Windows to be played instead of playing the file itself will also be unable to play the file. Note that Winamp and WMP play the file themselves and aren't going to experience the issue. Earlier in the thread, one thing was mentioned that uses the method mIRC uses. I bet that if you test with that, you'll find it also cannot play the files. That would clearly show that it's not a mIRC bug.
All of that said, it would not be a bad idea to put a feature request in to have mIRC handle playing sounds/music differently so that if the codec that matches the extension doesn't work, it will try some others just like programs like WMP do. That would reduce problems where a file is named incorrectly. That isn't what your problem appears to be, but it still isn't a bad feature request. Beyond that, though, it's not really a mIRC issue.
