You can auto play music on Facebook, but it requires access to a Facebook page. Regular Facebook profiles don't have the ability to add custom tabs. But if you own or manage a fan page on the social networking site, then you can use a simple HTML embed code to auto start a song. Keep in mind that Facebook frowns upon this type of practice, so use it sparingly and give the user an audio control panel to stop the song's playback.
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Go to facebook.com, sign into your profile and type "Static IFrame Tab" in the top search box. Select the app listing with the same name as the keywords. On the next screen, click the "Install Page Tab" button.
Click the "Add This Application To" drop-down menu and highlight the page you wish to add the custom tab to. Click the "Add Static IFrame Tab" button below. Your page loads with the new tab installed on the side.
Click the "Welcome" tab the app generated, then click the "Allow" button in the pop-up window.
Copy and paste the following HTML code into the "Enter the HTML for Your Page" text area: