I'd suggest another command then /play cause /play is a mIRC command alreaddy.

Anyways ... you want others in the channel to play the sound if they have it ?

If yes, the command is
/sound #CHANNEL "filename" message

Filename can be anything mirc can play, wav mp3.
If a user sees this sound request (and have it ON) and have the correct file in their SOUND directory (in mIRC) it will play for them aswell. (the "" if for any spaces in the file)

It will look something like ths....
/sound #CHANNEL "beat.wav" hums to the beat

* slartibartfast hums to the beat

So just call the /sound from an alias, it that is what you want.