The easiest way to transfer the files is through Dropbox or any other file sharing service. If you can access the MP3 file using File Manager, then open Browser application and navigate to Google Drive, Dropbox, Zippyshare, or any other file sharing service. I recommend Dropbox because it's fast, secure, and flexible.
Install the application from the available store, create a simple account and upload your files then you can use the browser on your desktop to log-in to Dropbox website to download them.
Another solution is to use HFS on your desktop. HFS is a lightweight HTTPD service with a very simple interface. Download the EXE file, launch it and then right click in the tree menu and add a folder from disk. Right click the folder and go to Properties > Upload > Check "Anyone".
Switch to Bluestacks and start Browser, enter the IP address from HFS then click the TEST folder. On the left menu select Upload > Choose your files and click Upload and your files should be found in the Test folder.
Additional info in the image below: