I kind of got around that by assuming that all songs could be described by a set length string, but that many of those would include sections of silence. That seems more straightforward to me than considering how many different lengths there could be and then summing each. Indeed, if you tried to do that, you'd then have to subtract any song-portions or songs-plus-silence where they were duplicated.

Er, that's if you were trying to get an accurate number, which I suppose we're not... just an order of magnitude. I think.