You know the answer to this

Please bare with me, I just started doing AS about a month ago.
I have a small graphical player and I want to load an external mp3 file into it. Now once the mp3 file is playing, I want the Play button to not play the same file again. Maybe these pictures will explain better what I mean…

the code is working in this pic:

the code is not working in this pic:

I know, I can use “removeEventListener” instead and it will work the way I want it to, but I wanted to know if some expert here could explain to me why this is giving me this error.

thank you all in advance :),

Aeson