With Apple’s iTunes service an unqualified success in the lucrative music-downloading business, the company is setting its sights on a massive expansion of the library of music available digitally. Consumers are coming to expect that iTunes will be able to come up with the music they want to hear, even if it’s obscure or out of print, and the company’s team of song-hunters are determined to convince record companies to open their vaults and license more songs for digital release.