With all of this talk about the webcasters owing new royalty rates to SoundExchange on May 15th, it's useful to remember that in the first quarter of 2006, the organization disbursed only about 60% of what it collected to the labels and artists it purports to represent. SoundExchange collected approximately $14.2 million in royalties and paid around $8.5 million to artists and labels.
SoundExchange says the difference is "held in reserve for artists and soundrecording copyright owners (SRCOs) that have not been identified orlocated." (If you think SoundExchange might owe you money, search for your song here; even if you don't want to charge for a song, SE is collecting royalties for it, so you might as well claim them.)
SoundExchange is only required to keep records for three years, so if you own a sound recording copyright -- defined as an actual recording, rather than a composition, which belongs to the song's publisher -- and register in the future, you'll only be paid for the previous three years.