license for slideshow

Hi

I do travelogue slideshows for various community groups, typically 20-30 people in the audience using Adobe lightroom and charge an hourly rate. 

I would like to have some background music while showing the pictures.Two questions:

1. what would be best program to download, google music, spotify, or something else?

2. Would I need a license, and if so how would i get one? If it’s complicated or expensive I won’t bother. 

Thanks