A welcome presentation may need music while people arrive, followed by slides containing spoken explanations or a video. Decide where background music helps and where it competes with the message before adding the same audio file to several slides.

Make a working copy and choose a short, permitted test track with an easily recognised opening. List any slides with their own sound. This gives you a specific place to listen for an unwanted restart or an overlap that makes speech difficult to understand.

In Keynote on Mac, open Document, Audio and add songs or playlists to the soundtrack. A soundtrack begins when the presentation starts and each soundtrack file plays from its beginning. Choose Play Once to stop when the soundtrack finishes, Loop to repeat until the presentation ends, or Off to disable it.

The soundtrack also plays on slides that contain their own video or audio. For playback elsewhere, Apple directs users to enable “Copy audio and movies into document” in Keynote Settings, General. DRM-protected music cannot be added as a soundtrack; use media you are entitled to include and play.

Listen through the actual slide boundaries

Start from the beginning and advance through several slides at different speeds. Listen for whether the musical phrase continues as intended. Then remain on one slide longer than usual so you can evaluate what happens when the soundtrack reaches its end.

For a looping welcome deck, listen across the repeat point. A restart that is technically correct can still be distracting if the track ends abruptly. Choose material and a playback plan that suit the waiting period rather than assuming the Loop setting makes any recording sound continuous.

Play the slide containing speech or a video at the expected output volume. Judge intelligibility from the audience position or a receiving test device. If the two audio sources compete, revise the soundtrack plan before the session instead of expecting the speaker to talk louder.

Verify the presentation copy on its destination

Open the actual deck on the computer that will present it and repeat the beginning, transition and overlapping-media checks. Confirm that the intended track is available and that the output volume is suitable for the room. Keep the editable deck and source-media information together so a later revision can reproduce the accepted soundtrack without accidentally selecting a different recording.

Reference: Apple: audio and presentation soundtracks in Keynote.