Asset Folder is readable folder you can't write or store files from SD card to assets folder. You can add some Mp3 files to your asset folder initially before preparing final build of your projects and can copy them to SD Card later.
Now you can access these files from SD Card or assets. You can store Mp3 songs from SD card to server only using Some RESTful POST web services.
Here is the link to copy files from assets to sd card
How to copy files from 'assets' folder to sdcard?
Hope it will help you