Most children with autism resist change and prefer sameness. Rather than just a stubborn habit, this is probably due to the child's inability to fully comprehend and pay attention to all the cues from their environment that announce a change is upcoming. Many children with autism focus on only a small part of the situation rather than the whole. Children with autism need to know what activity is first, next, and last. Children with autism should be warned about visitors coming into the classroom and home.
Social stories can be a great way to prepare children for changes and new situations.
Raising your voice seems like the natural thing to do to get compliance. However, many children with autism may be sound-sensitive. They may just avoid you. Also, when we raise our voice to a child, we have probably already gave the command four or five times. What we have taught the child is that the only time we really mean business is when we raise our voice.