You can add a song to a presentation and play it across slides in the background of your slide show. If you want to use music from the web, you have to download it to your computer before you can use it in a presentation.
MacOS
Add audio to a single slide
In Normal view, select the slide you want and click Insert > Audio.
Click Audio Browser to insert audio from iTunes, or Audio from File to insert an audio clip from your computer.
Select the audio clip. On the Audio Format tab, select the audio options you want.
(Optional) If you want to change or modify the standard audio file icon, use the picture format buttons on the Audio Format tab to add a frame, border, or other formatting effect to the audio icon.
On the slide, select the audio icon, and then click Play/Pause beneath the audio icon.
By default, during a slide show, audio plays when it is clicked. You can change it so that the audio plays automatically as soon as its slide appears.
In Normal view, select the slide you want and add an audio clip if you haven't already done so.
On the Audio Format tab, on the right, click Start > Automatically.
In Normal view, select the first slide in your presentation and add an audio clip if you haven't already done so.
On the Audio Format tab, on the right, click Play across Slides.
In Normal view, select the slide you want and add an audio clip if you haven't already done so.
On the Audio Format tab, on the right, click Loop Until Stopped.
(Used alone, this option means the looping sound lasts while the slide it resides on is being shown. When Loop Until Stopped is used in tandem with Play Across Slides, the looping sound continues throughout the presentation.)
Click the audio clip icon.
On the PowerPoint ribbon, on the Playback tab, select the Hide During Show check box.
Use this option only if you set the audio clip to play automatically. Note that the audio icon is always visible unless you drag it off the slide.
Play multiple songs across multiple slides
If your slideshow is longer than one song, you can add more songs. However, if you find that you're having trouble synchronizing the music with the slide show, you can use a third-party audio editing tool, such as Audacity®, to string the songs together into one file so they play continuously throughout the slide show.