A long time ago I issued a bugreport about /splay being unable to play a musicfile, while this file could be played in other players. I think I discovered what is going wrong. It seems mIrc is not able to play files with certain tags. I don't know which tag causes the problem, but if you remove all tags first and than add them again, mIrc can play the file! Try "Tag and Rename" to do this trick.

I have experienced this behaviour also, and have written some posts about it here.

On a sidenote, I'm currently making a dll, which is usable with mIRCs /dll and $dll(). The dll removes and/or rewrites the tags of a specified music file. This comes handy when creating a mp3 script for example.