Nah, don't even try when you know nothing about it, it will just ruin everything trust me. Just pick a good music probably in youtube, contact the channel and let them know that you are going to use their music to avoid getting taken down due to copyright issues and give link credits, that way you'll be fine.
Hey, that is a good idea, you know, he can invite that youtube channel to visit on their site, and maybe he can make a relationship or cooperation with them. I think that will be a new thing the gamblers will see on the gambling site, and I think that can attract more gamblers or visitors to come to that site. It is hard if it's related to the copyright issue because that can be a problem in the future if we ignore that thing.