Royalty-free can also apply to works of a composer who is a member of a collecting society (e.g., ASCAP, GEMA, SUISA). Royalty-free means that the publishing rights (i.e., synchronization, adaptation, and (film) production rights), and especially ancillary copyrights to finished productions, have been clarified. However, if these works are registered with a collecting society, PRO royalties must also be paid.
Since all audiohub songs are both royalty-free and non-P.R.O., there are no follow-on costs.