How can I add background music to an existing Drupal 7 website?
I'm looking at doing the same thing for a band's website where the music can keep playing even when switching pages. AJAX is definitely the way to go here.

Check out the pjax for Drupal module.


  pjax is a jQuery plugin that loads new content into the current page without a full page load. This is usually quicker and smoother than a full page reload, because Drupal only has to deliver the main page content and not the full page – and the browser only has to redraw a smaller amount of markup.
  
  In addition to this, pjax updates the page URL with the proper page path using HTML5 pushState – not with #url-fragments that are commonly used with ajax-loaded content. It also updates the page title with the title of the loaded page. In other words, it has all of the benefits of standard page loads (permalinks, SEO), while taking advantage of partial page loads in the browser.