Please advise me how to add bg music in dnn website?

you can do it with this free module:
Markit PageEar Module

When i tried to upload one mp3 file it's not accepting.

The File D:\Web Projects\TKH Website\Website\Portals\0\Templates\Mizhithammil.mp3 Is A Restricted File Type. Valid File Types Include ( *. swf, *.jpg, *.jpeg, *.jpe, *.gif, *.bmp, *.png, *.doc, *.xls, *.ppt, *.pdf, *.txt, *.xml, *.xsl, *.css, *.zip ). Please Contact Your Hosting Provider If You Need To Upload A File Type Which Is Not Supported.

please advice...

Go to the Host Settings page and add mp3 as an allowed file type.