If I understood correctly, you're wanting to echo a message to your status window once a /play has completed?
There is already an event for this, and here is the information directly from the mIRC help file:
on PLAYEND
The on PLAYEND event triggers when the /play command has finished playing a file.
Format: on <level>:PLAYEND:<commands>
Example: on 1:PLAYEND:/echo The play command has finished playing $filename
Note: Text files can be played ie. sent to users or channels on IRC by using the /play command.
