When I pitched my start-up at some competition, for example, I'd memorize everything rather than do it spontaneously. Interestingly, I stuttered much less or did not stutter at all and I suspect it was because I focused more on recalling the whole words than my stuttering. So is it possible for you to memorize some lines during the audition? I never auditioned before so I do not understand the context and how it goes. But my advice would be to try to come up with filler words in front of so that you get into the groove then afterward you can do it rather spontaneously. Goodluck!