NEW YORK — A young R&B singer auditioning at Jam Master Jay’s music studio described how her appointment with fame ended in horror — her songs interrupted by the gunshots that ended the Run-DMC star’s life. Wearing a tartan plaid bodysuit and her blonde dreadlocks tied in a bun atop her head, Yarrah Concepcion wept Thursday as she described how she and Jay’s friend Uriel Rincon tried to check the fallen DJ for a pulse after the fatal shooting. “When his arm moved, his brains — I’m sorry, I can’t talk about stuff like this. His brain stuff came out of his head,” she said. “I just started dry-he…