Several experts in digital music say that the music locker business is still legally ambiguous. For example, though some companies let people upload their music and listen to it elsewhere without any outcry from the labels, others, like MP3tunes, have been sued by music labels. Another issue: it is impossible for Web companies to tell whether a song was bought legally or downloaded illegally. Amazon says it has sidestepped the problem, because its users would upload their songs, in MP3 or A.A.C. format, to the cloud-based service, just like backing them up on an external hard drive or a Web-based computer backup service. “We don’t need a license to store music,” said Craig Pape, director of music at Amazon. “The functionality is the same as an external hard drive.”