You need to create a powerpoint as part of a parent session. You have lots of images of students doing activities in the classroom and on excursions. You have found a fantastic piece of music (creative commons as you will be playing to an audience) and you want it playing in the background of your powerpoint.
Find creative commons music and then save the piece of music to your own PC.
If using Powerpoint 2013, here are instructions and a video to help with adding audio to your powerpoint.
Microsoft gives even easier instructions for playing across all slides using Powerpoint 2013 or 2016.
Earlier versions of powerpoint have instructions here, but they are similar to those mentioned above.