Prime Music goes live Thursday with more than 1 million tracks pulled from the catalogs of two of the top three record labels - Warner Music Group and Sony Music - as well as from large independent labels. Amazon launched a streaming-music service called Prime Music, making the gamble that adding on-demand tunes to its $99-a-year Prime service will be more important in drawing new members than matching the exhaustive catalog offered by rivals Spotify and Apple's to-be-acquired Beats Music.