mIRC passes the filename directly onto Window using MCI (Media Control Interface), which then handles the playing of the file. Basically saying Windows Media Player or any 3rd party Media Players doesn’t play that of which mIRC’s /splay’s, all gets sent directly to the MCI Codec and then decompressed in the memory for playing.
https://forums.mirc.com/showflat.php?Cat=...amp;amp;fpart=1 
