There is much more to Selective Mutism that the inability to speak. Each individual is different and SM effects us all differently. Some people with SM will have difficulties with non-verbal communication, sensory processing, eating or doing other activities in front of others.
This is because Selective Mutism is an anxiety disorder. The points listed above can all be anxiety-provoking, hence why doing suck tasks can be even more difficult for people with an Selective Mutism.
When my anxiety is heightened, I find it difficult to type in order to communicate. I find it hard to show pain, answer questions by nodding or shaking my head and I find it difficult to eat around people I don't know. Many others with SM will also find these things challenging, some may find them impossible and others may find them easy.