Faq/Impress/102/da

How can I have a sound file play throughout a presentation?
To have a sound file play throughout the presentation: The sound will continue playing until another sound file is opened.
 * Select the slide where you want it to start playing;
 * or in the Slide pane, click Slide Transition;
 * On the Sound dropdown list (under Modify transition), select Other Sound.... A dialog box will open.
 * Browse to the desired file and open it.

See also this question.