The thing is that I want to put a background music in my PPT like even if I change the slide I dont?

1.Open your presentation and click the first slide in the left Slides panel.
2.Click the "Insert" tab. Click the arrow under "Sound" in the Media Clips section and select "Sound From File."
3.Double-click your music file in the Insert Sound navigation window.
4.Click "Automatically" in the pop-up window that asks how the sound should start.
Click the "Options" tab.
5.Check "Hide During Show" and "Loop Until Stopped" in the Sound Options group.
6.Click the "Play Sound" drop-down menu from the Sound Options group and select "Play Across Slides."
7.Click anywhere in the slide to exit the sound options. The music is then set to start automatically from the first slide and continue playing throughout the entire presentation. If the presentation lasts longer than the song, the music will repeat.

I hope this helps you.