Hi there and welcome Michael.
To do this
On the Insert tab, select Audio, and then Audio on My PC.
In the file explorer, locate the music file you want to use and then select Insert.
On the Playback tab, select Play in Background.
This then plays it in the background over all slides.
If you have any problems, or would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.