Sources on set for 'XF' auditions in Greensboro, NC ... tell us the two guys laid down a rap that included the line, "I'm high on bath salts."

Apparently most of the crowd didn't hear the lyrics, but Demi did -- and as soon as the duo finished she furiously scolded them, saying ... "It's really inappropriate to rap about drugs in front of kids."

Demi did a stint in rehab in 2011, and has admitted to using drugs and booze when she was underage -- so, it's no wonder she went off on the clowns.

We're told the audience agreed with Demi, and booed the contestants off stage ... and they were not judged for the competition.