The Custom Music Database is a website full of already recorded, edited and looped custom music ready to be used in your level! In case you don't feel like downloading a song, editing it your own, and searching a host to upload it, this site is the perfect solution - all of that is already done for you!
There's a varied amount of custom music on the site - you can sort them all by category/theme (for example grassland, cave, volcano, etc). There's even music for events - music for certain sections of a level - for example, raising lava sections or a section where you have to be really careful.
Remember, you can also check out the custom music tutorial for help adding music to your level!