The crying out with a loud voice was a confirmation that a miraculous liberation from unclean spirits had taken place. If there was no accompanying sound, then people would have no reason to believe that the unclean spirits had left those who were affected by them.1
7 Car des esprits impurs sortirent de plusieurs démoniaques, en poussant de grands cris, et beaucoup de paralytiques et de boiteux furent guéris.