“For some time, DJs – at first in England and later in the United States and other countries – have been developing a new style of remix, known as mashups, in which two songs are melded together. Often, the resulting track features the melody of one song and the vocals of another. Until recently, mashups had been the province of underground DJs, in part because those doing the remixing hardly ever had permission from the original artists to do so.” But now, David Bowie has issued a mashup challenge…