We use royalty free music in our projects and below are few samples available under Creative Common license and it is Free to use, even in commercial projects but you must give credit to the musicians.

if you wish to add your any music, you can buy it yourself and send us the file to include in your voice over.
Further, you can also choose to youtube audio library but required to credit its owner.
Please review youtube music here.
https://www.youtube.com/audiolibrary/music