Tension hoops are a great, and cheap, tool to use in speech. You can use them with students who are working on auditory memory, phoneme discrimination, listening comprehension, and more.
Just grab a tension hoop and some dark cloth. The cloth will block your face without impeding the sound of your voice. It can challenge students to use their skills without reading lips or watching your face for cues ( if those are behaviors you are looking to diminish, of course ). I found this tension hoop with the sewing materials at Jo-Ann Fabric. It was $1.50, not bad.
You can watch this quick video if you want to know more 🙂