My family are friends with one of the women who helps to run SMIRA, and I've helped out at the annual SMIRA meetings for the past 3 years. I help look after the children while the parents ask for advice and meet others who have the same problems. Because the children are so painfully shy, you have to try hard to get them to join in with games, or interact with other children. Some react better than others.
It's very upsetting to observe but it is curable - generally the younger a child is, the easier it is to overcome. Some of the older/worse affected children are not only SM, but have OCD and/or autism, which complicates things. Some siblings, especially twins/triplets, will ALL be silent, but others will have a sibling who is much louder and speaks 'for' them. The family dynamics are very interesting. And interestingly, bilingual children often seem to struggle, though I'm not sure research through SMIRA has proved any direct link yet.
I know what it's like to be shy and I suffer from panic attacks, so I empathise. The more people are aware of SM, the more research is done on it, and the better the reaction is from teachers and doctors. I've heard about some dreadful experiences parents have had, trying to get the condition recognised and treated properly. It's so much more than just 'being shy'!