First, blame the band. Then call it a computer glitch. And finally, the best justification: acid reflux disease. Ashlee Simpson, 20, spent the week doing damage control after her Oct. 23 Saturday Night Live performance revealed a prerecorded vocal track, and the embarrassed singer fled the stage. "It was something I had never done and I didn't want to do," Simpson (with host Jude Law) told PEOPLE of lip-synching. "(But) my voice wasn't strong enough."