Difference between Royalty Free and Non Royalty Free Sites.
This is often a confusing term between different publishers. Generally any music published on a royalty free site and classified as 'Non P.R.O Music' will not generate performance royalties further down the line. Sites like Audio Jungle present this as a benefit to their customers, stating that once they pay the initial licence fee, the customers are not liable for any more costs related to the track that they have licensed.
This however does not mean that royalties cannot be generated from other 'Royalty Free Sites'. Indeed, even Audio Jungle now offers 'P.R.O registered music'. If a licensed track has made its way on to say a television episode that has been broadcast, if that track has been registered with a Performing Rights Society such as BMI or ASCAP, then the Broadcaster would be required to pay a royalty for that usage.
In reality, music published on a stock site such as Audio Jungle or Pond5 rarely generates any substantial royalty income, as their usages are generally for the vast array of production mediums outside of television.