Online music sales have leveled off in the course of the past year, Nielsen said today. Sales and subscriptions at iTunes, Amazon MP3 and other American stores were roughly similar to 2009 compared to a 13 percent increase the year before and a 28 percent jump in 2008. The researchers didn't have a direct explanation but suspected that disenchantment with the music on sale, as well as economic difficulties and a confusion of sources, contributed to the effect.