I’d like to point out that “royalty free” is not the same as “public domain” (or CC0), so please make sure to check the licences when using any music from ‘royalty free’ sites.
In many cases you still have to provide credit to the author to properly adhere to the licence.
I don’t say this just to be pedantic, people have given up their own time to create this music,
the least they deserve is a bit of credit.
(If you don’t want to give credit, you can usually buy the right to not have to provide attribution.)
(Most music is CC BY, which is similar to the Apache 2.0 licence for code,
i.e. you can change it as much as you like,
but you must provide a copyright notice to say who the original author is,
and you must indicate if any changes were made.)
One other thing to note about “royalty free” is its not the same as “free”
Royalty free means you don’t pay royalties (portion of the sales of whatever product uses the music/sounds) but more often than naught still charges a flat fee per project to use the resources.
If you search “royalty free music” you’ll find the vast majority are either licensed out on a per project basis for a flat fee, or the site uses a subscription model to get “unlimited” access to use in any project.
Best bet is to search “creative commons music” or “public domain music” (depending on whether you’re ok with proper attribution).
IMHO: even public domain works should still deserve some kind of nod to the original author/artist for they’re hard work and efforts.
Each year we give away thousands of dollars worth of sound effects for free in celebration of the Game Developers Conference. This is our archive. Everything is royalty-free and commercially usable. No attribution is required and you can use them on an unlimited number of projects."