If Dropbox has picture viewer, why can't it have audio player?
Dropbox could provide audio player for private files.
Although I think the real comparison is between Amazon and bigger players like iTunes and Google Drive.
Seems like that would throw an interesting twist into the music/movie copyright battles. You could in theory upload and consume a lot of content that you never really 'uploaded'.
This is definitely true. For example, if you put a video show you got off of eztv in your dropbox, the upload is instantenously complete. Why? because someone else has already put that file into dropbox so you don't need to upload it as well.