As far as I understand, a song is copyrighted as soon as it's written and as long as you can prove you wrote it, you should get paid whenever it gets played anywhere and companies like ASCAP and BMI in the States and PRS in the UK collect those payments on your behalf once you become a member. This is why every business that plays music in a public space needs a PRS license and any live gig you do over a certain size will ask you to fill in a PRS form (I don't know how that bit works in the states). Uploading your music to a value for value platform is almost like licensing your music for free so anyone can play it or listen for free, as long as any value that is sent is finding it's way back to you, then it's a voulantary choice and opt in. If a singer sang a song written by somebody else, they would just have to privately negotiate that value split on a V4V model. I can personally say I have made more from V4V this year than I have had from PRS all previous years combined (excluding royalties for live performance)

Hope this helps