use audio over more than one slide

Take a look at the Audio tab (under Document), specifically "Soundtrack." This is probably what you want.

From the Help:

Add a soundtrack to a presentation

When you add a soundtrack to your presentation, the audio begins playing when the presentation starts. If there are slides that already have audio or video, the soundtrack also plays on those slides.

A file added as a soundtrack always plays from its beginning.

1. Click the Audio tab at the top of the sidebar on the right.If you don’t see a sidebar, or the sidebar doesn’t have an Audio tab, click in the toolbar.
2. Click , then select a song or playlist.
3. Click the soundtrack pop-up menu, then choose an option:
   • Off: The soundtrack doesn’t play.
   • Play Once: The soundtrack plays through the presentation and doesn’t repeat if the presentation is longer than the soundtrack.
   • Loop: The soundtrack repeats until the presentation ends.

If you want to play your presentation on a different computer or device, choose Keynote > Preferences (from the Keynote menu at the top of your screen), click General, then select “Copy audio and movies into document.” Doing so ensures the soundtrack is available whenever you play it.

Some media files are protected under copyright law. Some downloaded media may be played only on the device where the download occurred. DRM (digital rights management) protected music can’t be added to the presentation soundtrack. Make sure the computer you’re using has permission to play all the media files included in your presentation.

The only way to have music playing over a selected group of slides in Keynote, is to place all the content, (including the sound file) to be played with the sound file on a single new slide. Use the build order to time and order the content: Inspector > Animate > Add an Effect; click the Build Order button.

I like Gary's solution but perhaps we still don't have a clear idea of the effect that you are trying to produce. This is a manually controlled presentation (correct?) and you have an audio clip (music? sound effects?) on slide one and you want a soundtrack to kick in after slide one and continue for its duration w/o looping. The idea behind a soundtrack is that the audio is continuous and not linked to specific slides. You don't want audio to start and stop in sync with your manual control of the slides (correct?). If that's the case, then you could use GarageBand to assemble a soundtrack that is silent for the first n minutes where n is the duration of that audio clip on slide one.

An auto playing slideshow would present a different set of challenges and require different techniques.

Gary thanks but i cannot seem to get the slides i want into build

Copy thecontent, (images, text, shapes and media files) from each existing slide and paste into the new single slide.

Then use the build order pane to time and order the each item.

Cut all the objects from the existing group of slides that will accompany the sound (images, text, shapes and media files) and paste into a new slide.

Obviously delete the existing slides when finished pasting.

You could consider this; it would take me less than 2 minutes of my time to do this. If you send a Keynote file containing only the slides with the images that are to play with the music and the slide containing the music I can put this together for you by return.

If you want this done send me the file using dropbox. Dropbox

My email is in my profile

If on the other hand you want to learn how to do this yourself I am happy to send detailed instructions for the next step.

Good to hear.