The leading DRM digital download service, Apple's iTunes, has experienced a collapse in sales revenues this year according to analyst company Forrester Research. Secretive Apple doesn't break out revenues from iTunes, but Forrester conducted an analysis of credit card transactions over a 27-month period. And this year's numbers aren't good.[...] And it isn't just Apple's problem. Nielsen Soundscan has grimmer news for prospective digital download services, indicating three consecutive quarters of flat or declining revenues for the sector as a whole.