You can find a lot of free stock music (but at the risk to use a well known music).
There’s several musician how put music on the market place.
You could also contact some musician you like on soundcloud and ask them to use their work, a lot of it are CC but if not, there’s chance they let you use it if you ask politely 
When you come to free stuff, be aware that the copyleft license is not compatible with UE4 EULA.