By default, mIRC supports playing .mp3, .wav, and .midi sound files. There has been some discussion as to whether mIRC uses the extension to identify which kind of file is being played, or if it uses the actual file format (although how it would do this is beyond my knowledge).
You can easily test if mIRC will play your sound files, by typing /splay <filename>
The answer to your question will quickly be answered by doing that.
I am unable to test it myself, as I don't have any audio files with that extension.
Additionally, even if I did, if mIRC uses the file format to determine how it is to be played, rather than the file extension, there's no way for me to know that the .au file I had (or created) matched the format that your files are in.
