BMI is a US organisation. Since I assume from pavane's earlier question that he's in the UK, PRS/MCPS would be the equivalent. However I also think he's talking about paying royalties to other composers, rather than being paid for his own songs.

I recently put an album by my former band (the Electropathics) online with CD Baby. We'd already covered copyright with MCPS for the original vinyl release, and when I contacted them to ask whether we needed to pay another fee for online release, they told me this is covered by the fees paid by the online sellers.

http://www.cdbaby.com/cd/Electropathics