How to play sound located in different folder in flex?

I can access and play file named "Song.mp3" located in the root directory of my project but when I put it into "assets" directory I got error.

Here is my project directory:

-Test
-- src 
---Homeview.mxml 
-- assets 
---Song.mp3 
--Song.mp3

I can access and play Song.mp3 located in the root folder but I get error when I want to play assets/Song.mp3.

This works:

var req:URLRequest=newURLRequest('../Song.mp3');

This does not:

var req:URLRequest=newURLRequest('../assets/Song.mp3');

I get following Runtime error:

Error#2044: Unhandled ioError:. text=Error #2032: Stream Error. URL: app:/assets/Song.mp3

I think that Song.mp3 in the root directory get exported in the final swf file but assets/Song.mp3 does not. If that is the case is there a way to force Flex to export assets/Song.mp3?