By the Numbers: NIH on Quick Statistics About Communication Disorders(link is external)
Best Practice: Allow for Augmentative and Alternative Communication device input to XR systems.
Example: LetMeTalk (link is external)is a free app that can run on a user's smart phones or tablet while providing vocalized speech audio input to a primary computer.
Further Reading: Tobii Dynavox on Getting Started with ACC(link is external)