Hello all and sorry for the late response, this a great conversation, however, I believe there might be a bit of misconception on PRO's in regards to the role they play with composers and filmmakers. The main role of P.R.Os is to collect and distributes money on behalf of songwriters, composers, and music publishers, for the use of their musical compositions in media.
A misconception we come across a lot in my field of the music industry is that a lot of Filmmaker believes they are responsible for paying royalties to the composer, which is why they tend to gravitate to "Royalty-Free Music" which is completely wrong. The P.R.O is the only entity responsible for paying royalties to the composer should your film end up on TV!
In regards to licensing, copyright, and publishing agreements mentioned above. Those are all 3 different beasts that are not related to film making. Copyrights are registered by whoever created the music or whoever controls the music. This protects the artist should anyone ever try to steal or copy their music. Publishing agreements are only handled by music publishers. A publishing agreement is an agreement that grants a party the permission to solicit your work to different opportunities on your behalf. Lastly, licensing falls under publishing. This is a term used when the artists or composer is allowing their music to be used with some kind of visual media i.e film, television shows, advertisements, etc...
A lot of information I know! That said, the point I want to get across is that when you're looking for music for your films, the only compensation you should be paying out is sometimes a sync fee to the composer/artist which is an upfront fee paid for the permission to use the composition to moving images. This fee is needed because unless the film ends up on TV there will be no income for the composer to make. In regards to royalties, the P.R.O has a great system that monitors music and will pay the composer/artists the appropriate royalties should any ever be generated.