(nyt) If you’re looking for a two-word motto for hip-hop in 2007, you could do worse than that: “Keep grinding.” This was the year when the gleaming hip-hop machine — the one that minted a long string of big-name stars, from Snoop Dogg to OutKast — finally broke down, leaving rappers no alternative but to work harder, and for fewer rewards. Newcomers arrived with big singles and bigger hopes, only to fall off the charts after selling a few hundred thousand copies, if that. Hip-pop hybrids dominated the radio, but rappers themselves seemed like underground figures, for the first time in nearly two decades. Sales are down all over, but hip-hop has been hit particularly hard. Rap sales fell 21 percent from 2005 to 2006, and that trend seems to be continuing. It’s the inevitable aftermath, perhaps, of the genre’s vertiginous rise in the 1990s, during which a series of breakout stars — Dr. Dre, Snoop Dogg, Tupac Shakur, the Notorious B.I.G. — figured out that they could sell millions without shaving off their rough edges. By 1997 the ubiquity of Puff Daddy helped cement hip-hop’s new image: the rapper as tycoon. Like all pop-music trends, like all economic booms, this one couldn’t last.