Depending on the specific child, WBT can be really great for students with autism. I've had personal experience with (and had colleagues who were) using gestures with students who had autism spectrum disorders (both very different from each other: one was very severe who needed a full-time aide, and the other was very mild and talked in class but had a hard time sharing out to the whole class)
The gestures were particularly helpful to both of them, but the one with the more severe autism had a harder time with the loud noises.
If you are using WBT with the majority of students for the first time you can simply tell them how loud to say things right off the bat. I've used a "whisper WBT" during testing week and if they have a good reason to be quiet then they are quiet(er) than normal. Use the smiley/frownie chart to your advantage, if they are a little too loud, then give 'em a frownie and have them practice at a more acceptable tone/noise level. You could also use the volume-meter (yadayadayadayada from silent to frenzy). Pair this with some empathy and explanation about what autism is and how it affects persons with it (for example touch and sound is perceived very differently in some with autism), and you should be able to adapt it well for your students.
Students with autism benefit greatly from structured routines and steps/procedures. WBT is so full of structure, routine, procedures, and practice (as opposed to rules and punishments), it is great for students who have experienced ABA therapy (a form of therapy used for children with autism that is very procedure/routine based.)
Let us know how it goes!