Client should upload mp3 file to the custom file manager and then add following html code to their site :

<embed hidden="true" src="" autostart="true" loop="true"></embed>

(name of the file instead of file_name.mp3)

They can add code to slogan or footer (in that case music will be played on homepage)
If they wants to play it on all pages they can add it to additional head content.

Please let us know if we should do it for the client (please ask for their mp3 file in that case)