By default there is no sound during a picture slideshow. You can add a specific song from iTunes to play while watching pictures.
To get started:
• Make sure the sharing checkbox is enabled in the Sharing pane.
• Click the "Change Song" button in the Pictures pane.
• Scroll or search for the song you want and select it.
• For AAC files, the songs will be prepared for streaming first and cached.
Learn more: Media Sharing, AAC Music Files.