It's depend the version of powerpoint you are using.
How do you have music play throughout your whole PowerPoint?
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PowerPoint presentations can be designed to advance manually.
Sometimes, when you complete your presentations, you may also need to
automate them and set up the exact time duration for each slide and so
that they can run by themselves. How do you have music play throughout
your whole PowerPoint?
First, download the song(s) in mp3 or WAV format that can be accepted
by power point.
If you have PowerPoint 2007:
1>insert the music file:
go to insert >sound>Select Sound from File ...choose the sound which
you want ,Click OK and a message prompts, saying How do you want the
sound to start in the slide show? Click Automatically.
2>Set up the effect of the sound:
1. click animations tab, then click the custom animation, you can see
a pop-up window.
2. Click the sound file that you want to set, Click the drop down
arrow of the music item you just added from the Custom Animation list
and select Effect Options.
3. A window named Play Sound displays. Select After and then set the
total number of slides .Now when you select OK and exit the custom
animation box, your sound will continue to play.
If you have PowerPoint 2003:
1>insert the music file:
click the sound icon to display the Options tab. Click on the tab,
and set Play across slides for Play Sound.
2>Set up the effect of the sound:
1.right click on the icon in your presentation and select "Custom
Animation."
2. There will be a section for "Effect" with the option to stop
playing after X slides.
3. Set the number of slides for the sound to play here. Now when you
select OK and exit the custom animation box, your sound will continue
to play.
For more details just check it out below:
http://www.ppt-to-dvd.com/learningcenter/how-to-add-music-song-to-powerpoint-presentations.html#149